fix(bedrock): return string content in ChatBedrockConverse streaming

Author: SYED-M-HUSSAIN

libs/aws/langchain_aws/chat_models/bedrock_converse.py

@@ -1170,9 +1170,9 @@ def _parse_stream_event(event: Dict[str, Any]) -> Optional[BaseMessageChunk]:
     if "messageStart" in event:
         # TODO: needed?
         return (
-            AIMessageChunk(content=[])
+            AIMessageChunk(content="")
             if event["messageStart"]["role"] == "assistant"
-            else HumanMessageChunk(content=[])
+            else HumanMessageChunk(content="")
         )
     elif "contentBlockStart" in event:
         block = {
@@ -1189,7 +1189,9 @@ def _parse_stream_event(event: Dict[str, Any]) -> Optional[BaseMessageChunk]:
                     index=event["contentBlockStart"]["contentBlockIndex"],
                 )
             )
-        return AIMessageChunk(content=[block], tool_call_chunks=tool_call_chunks)
+        # Keep content as list during streaming 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 +1207,19 @@ def _parse_stream_event(event: Dict[str, Any]) -> Optional[BaseMessageChunk]:
                     index=event["contentBlockDelta"]["contentBlockIndex"],
                 )
             )
-        return AIMessageChunk(content=[block], tool_call_chunks=tool_call_chunks)
+        # Keep content as list during streaming 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]