core[major]: On Tool End Observation Casting Fix (#18798)

This PR updates the on_tool_end handlers to return the raw output from the tool instead of casting it to a string. 

This is technically a breaking change, though it's impact is expected to be somewhat minimal. It will fix behavior in `astream_events` as well.

Fixes the following issue #18760 raised by @eyurtsev

---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

docs/docs/modules/agents/how_to/streaming.ipynb

@@ -1068,7 +1068,7 @@
     "\n",
     "    def on_tool_end(\n",
     "        self,\n",
-    "        output: str,\n",
+    "        output: Any,\n",
     "        *,\n",
     "        run_id: UUID,\n",
     "        parent_run_id: Optional[UUID] = None,\n",
@@ -1076,7 +1076,7 @@
     "    ) -> Any:\n",
     "        \"\"\"Run when tool ends running.\"\"\"\n",
     "        print(\"Tool end\")\n",
-    "        print(output)\n",
+    "        print(str(output))\n",
     "\n",
     "    async def on_llm_end(\n",
     "        self,\n",

docs/docs/modules/callbacks/index.mdx

@@ -59,7 +59,7 @@ class BaseCallbackHandler:
     ) -> Any:
         """Run when tool starts running."""
 
-    def on_tool_end(self, output: str, **kwargs: Any) -> Any:
+    def on_tool_end(self, output: Any, **kwargs: Any) -> Any:
         """Run when tool ends running."""
 
     def on_tool_error(