fix: Return ChatBedrockConverse streaming content as str instead of list (#589)

Fixes #486

Supersedes PR #491 -
full credit goes to SYED-M-HUSSAIN for the original fix

---------

Co-authored-by: Hussainbeam <[email protected]>

Author: BumaldaOverTheWater94

libs/aws/langchain_aws/chat_models/bedrock_converse.py

@@ -1189,7 +1189,10 @@ def _parse_stream_event(event: Dict[str, Any]) -> Optional[BaseMessageChunk]:
                     index=event["contentBlockStart"]["contentBlockIndex"],
                 )
             )
-        return AIMessageChunk(content=[block], tool_call_chunks=tool_call_chunks)
+        # always keep block inside a list to preserve merging compatibility
+        content = [block]
+        
+        return AIMessageChunk(content=content, tool_call_chunks=tool_call_chunks)
     elif "contentBlockDelta" in event:
         block = {
             **_bedrock_to_lc([event["contentBlockDelta"]["delta"]])[0],
@@ -1205,19 +1208,20 @@ def _parse_stream_event(event: Dict[str, Any]) -> Optional[BaseMessageChunk]:
                     index=event["contentBlockDelta"]["contentBlockIndex"],
                 )
             )
-        return AIMessageChunk(content=[block], tool_call_chunks=tool_call_chunks)
+        # always keep block inside a list to preserve merging compatibility
+        content = [block]
+        
+        return AIMessageChunk(content=content, tool_call_chunks=tool_call_chunks)
     elif "contentBlockStop" in event:
         # TODO: needed?
-        return AIMessageChunk(
-            content=[{"index": event["contentBlockStop"]["contentBlockIndex"]}]
-        )
+        return AIMessageChunk(content=[])
     elif "messageStop" in event:
         # TODO: snake case response metadata?
-        return AIMessageChunk(content=[], response_metadata=event["messageStop"])
+        return AIMessageChunk(content="", response_metadata=event["messageStop"])
     elif "metadata" in event:
         usage = _extract_usage_metadata(event["metadata"])
         return AIMessageChunk(
-            content=[], response_metadata=event["metadata"], usage_metadata=usage
+            content="", response_metadata=event["metadata"], usage_metadata=usage
         )
     elif "Exception" in list(event.keys())[0]:
         name, info = list(event.items())[0]

libs/aws/tests/integration_tests/chat_models/test_bedrock_converse.py

@@ -455,7 +455,7 @@ def test_structured_output_thinking_force_tool_use() -> None:
         },
     }
     with pytest.raises(llm.client.exceptions.ValidationException):
-        response = llm.client.converse(messages=messages, **params)
+        llm.client.converse(messages=messages, **params)
 
 
 def test_bedrock_pdf_inputs() -> None: