standard-tests[patch]: require model_name in response_metadata if returns_usage_metadata (#30497)

We are implementing a token-counting callback handler in
`langchain-core` that is intended to work with all chat models
supporting usage metadata. The callback will aggregate usage metadata by
model. This requires responses to include the model name in its
metadata.

To support this, if a model `returns_usage_metadata`, we check that it
includes a string model name in its `response_metadata` in the
`"model_name"` key.

More context: https://github.com/langchain-ai/langchain/pull/30487

libs/cli/langchain_cli/integration_template/integration_template/chat_models.py

@@ -329,6 +329,7 @@ class Chat__ModuleName__(BaseChatModel):
             additional_kwargs={},  # Used to add additional payload to the message
             response_metadata={  # Use for response metadata
                 "time_in_seconds": 3,
+                "model_name": self.model_name,
             },
             usage_metadata={
                 "input_tokens": ct_input_tokens,
@@ -391,7 +392,10 @@ class Chat__ModuleName__(BaseChatModel):
 
         # Let's add some other information (e.g., response metadata)
         chunk = ChatGenerationChunk(
-            message=AIMessageChunk(content="", response_metadata={"time_in_sec": 3})
+            message=AIMessageChunk(
+                content="",
+                response_metadata={"time_in_sec": 3, "model_name": self.model_name},
+            )
         )
         if run_manager:
             # This is optional in newer versions of LangChain