perf[core]: remove costly async helpers for non-end event handlers (#31230)

1. Remove `shielded` decorator from non-end event handlers
2. Exit early with a `self.handlers` check instead of doing unnecessary
asyncio work

Using a benchmark that processes ~200k chunks (a poem about broccoli).

Before: ~15s

Circled in blue is unnecessary event handling time. This is addressed by
point 2 above

<img width="1347" alt="Screenshot 2025-05-14 at 7 37 53 AM"
src="https://github.com/user-attachments/assets/675e0fed-8f37-46c0-90b3-bef3cb9a1e86"
/>

After: ~4.2s

The total time is largely reduced by the removal of the `shielded`
decorator, which holds little significance for non-end handlers.

<img width="1348" alt="Screenshot 2025-05-14 at 7 37 22 AM"
src="https://github.com/user-attachments/assets/54be8a3e-5827-4136-a87b-54b0d40fe331"
/>

libs/core/langchain_core/callbacks/manager.py

@@ -520,6 +520,8 @@ class RunManager(BaseRunManager):
         Returns:
             Any: The result of the callback.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_text",
@@ -542,6 +544,8 @@ class RunManager(BaseRunManager):
             retry_state (RetryCallState): The retry state.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_retry",
@@ -601,6 +605,8 @@ class AsyncRunManager(BaseRunManager, ABC):
         Returns:
             Any: The result of the callback.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_text",
@@ -623,6 +629,8 @@ class AsyncRunManager(BaseRunManager, ABC):
             retry_state (RetryCallState): The retry state.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_retry",
@@ -675,6 +683,8 @@ class CallbackManagerForLLMRun(RunManager, LLMManagerMixin):
                 The chunk. Defaults to None.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_llm_new_token",
@@ -694,6 +704,8 @@ class CallbackManagerForLLMRun(RunManager, LLMManagerMixin):
             response (LLMResult): The LLM result.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_llm_end",
@@ -718,6 +730,8 @@ class CallbackManagerForLLMRun(RunManager, LLMManagerMixin):
                 - response (LLMResult): The response which was generated before
                     the error occurred.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_llm_error",
@@ -750,7 +764,6 @@ class AsyncCallbackManagerForLLMRun(AsyncRunManager, LLMManagerMixin):
             inheritable_metadata=self.inheritable_metadata,
         )
 
-    @shielded
     async def on_llm_new_token(
         self,
         token: str,
@@ -766,6 +779,8 @@ class AsyncCallbackManagerForLLMRun(AsyncRunManager, LLMManagerMixin):
                 The chunk. Defaults to None.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_llm_new_token",
@@ -786,6 +801,8 @@ class AsyncCallbackManagerForLLMRun(AsyncRunManager, LLMManagerMixin):
             response (LLMResult): The LLM result.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_llm_end",
@@ -814,6 +831,8 @@ class AsyncCallbackManagerForLLMRun(AsyncRunManager, LLMManagerMixin):
 
 
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_llm_error",
@@ -836,6 +855,8 @@ class CallbackManagerForChainRun(ParentRunManager, ChainManagerMixin):
             outputs (Union[dict[str, Any], Any]): The outputs of the chain.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_chain_end",
@@ -858,6 +879,8 @@ class CallbackManagerForChainRun(ParentRunManager, ChainManagerMixin):
             error (Exception or KeyboardInterrupt): The error.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_chain_error",
@@ -879,6 +902,8 @@ class CallbackManagerForChainRun(ParentRunManager, ChainManagerMixin):
         Returns:
             Any: The result of the callback.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_agent_action",
@@ -900,6 +925,8 @@ class CallbackManagerForChainRun(ParentRunManager, ChainManagerMixin):
         Returns:
             Any: The result of the callback.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_agent_finish",
@@ -942,6 +969,8 @@ class AsyncCallbackManagerForChainRun(AsyncParentRunManager, ChainManagerMixin):
             outputs (Union[dict[str, Any], Any]): The outputs of the chain.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_chain_end",
@@ -965,6 +994,8 @@ class AsyncCallbackManagerForChainRun(AsyncParentRunManager, ChainManagerMixin):
             error (Exception or KeyboardInterrupt): The error.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_chain_error",
@@ -976,7 +1007,6 @@ class AsyncCallbackManagerForChainRun(AsyncParentRunManager, ChainManagerMixin):
             **kwargs,
         )
 
-    @shielded
     async def on_agent_action(self, action: AgentAction, **kwargs: Any) -> Any:
         """Run when agent action is received.
 
@@ -987,6 +1017,8 @@ class AsyncCallbackManagerForChainRun(AsyncParentRunManager, ChainManagerMixin):
         Returns:
             Any: The result of the callback.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_agent_action",
@@ -998,7 +1030,6 @@ class AsyncCallbackManagerForChainRun(AsyncParentRunManager, ChainManagerMixin):
             **kwargs,
         )
 
-    @shielded
     async def on_agent_finish(self, finish: AgentFinish, **kwargs: Any) -> Any:
         """Run when agent finish is received.
 
@@ -1009,6 +1040,8 @@ class AsyncCallbackManagerForChainRun(AsyncParentRunManager, ChainManagerMixin):
         Returns:
             Any: The result of the callback.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_agent_finish",
@@ -1035,6 +1068,8 @@ class CallbackManagerForToolRun(ParentRunManager, ToolManagerMixin):
             output (Any): The output of the tool.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_tool_end",
@@ -1057,6 +1092,8 @@ class CallbackManagerForToolRun(ParentRunManager, ToolManagerMixin):
             error (Exception or KeyboardInterrupt): The error.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_tool_error",
@@ -1089,7 +1126,6 @@ class AsyncCallbackManagerForToolRun(AsyncParentRunManager, ToolManagerMixin):
             inheritable_metadata=self.inheritable_metadata,
         )
 
-    @shielded
     async def on_tool_end(self, output: Any, **kwargs: Any) -> None:
         """Async run when the tool ends running.
 
@@ -1097,6 +1133,8 @@ class AsyncCallbackManagerForToolRun(AsyncParentRunManager, ToolManagerMixin):
             output (Any): The output of the tool.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_tool_end",
@@ -1108,7 +1146,6 @@ class AsyncCallbackManagerForToolRun(AsyncParentRunManager, ToolManagerMixin):
             **kwargs,
         )
 
-    @shielded
     async def on_tool_error(
         self,
         error: BaseException,
@@ -1120,6 +1157,8 @@ class AsyncCallbackManagerForToolRun(AsyncParentRunManager, ToolManagerMixin):
             error (Exception or KeyboardInterrupt): The error.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_tool_error",
@@ -1146,6 +1185,8 @@ class CallbackManagerForRetrieverRun(ParentRunManager, RetrieverManagerMixin):
             documents (Sequence[Document]): The retrieved documents.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_retriever_end",
@@ -1168,6 +1209,8 @@ class CallbackManagerForRetrieverRun(ParentRunManager, RetrieverManagerMixin):
             error (BaseException): The error.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         handle_event(
             self.handlers,
             "on_retriever_error",
@@ -1213,6 +1256,8 @@ class AsyncCallbackManagerForRetrieverRun(
             documents (Sequence[Document]): The retrieved documents.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_retriever_end",
@@ -1236,6 +1281,8 @@ class AsyncCallbackManagerForRetrieverRun(
             error (BaseException): The error.
             **kwargs (Any): Additional keyword arguments.
         """
+        if not self.handlers:
+            return
         await ahandle_event(
             self.handlers,
             "on_retriever_error",
@@ -1521,6 +1568,8 @@ class CallbackManager(BaseCallbackManager):
 
         .. versionadded:: 0.2.14
         """
+        if not self.handlers:
+            return
         if kwargs:
             msg = (
                 "The dispatcher API does not accept additional keyword arguments."
@@ -1998,6 +2047,8 @@ class AsyncCallbackManager(BaseCallbackManager):
 
         .. versionadded:: 0.2.14
         """
+        if not self.handlers:
+            return
         if run_id is None:
             run_id = uuid.uuid4()