feat: use reasoning field in StreamingChunk for Bedrock (#2901)

Populate StreamingChunk.reasoning with ReasoningContent instead of
storing reasoning content deltas in meta["reasoning_contents"].
Update _process_reasoning_contents to read from chunk.reasoning.extra
instead of chunk.meta.

Author: Br1an67

integrations/amazon_bedrock/src/haystack_integrations/components/generators/amazon_bedrock/chat/utils.py

@@ -585,13 +585,15 @@ def _convert_event_to_streaming_chunk(
             reasoning_content = delta["reasoningContent"]
             if "redactedContent" in reasoning_content:
                 reasoning_content["redacted_content"] = reasoning_content.pop("redactedContent")
+            reasoning_text = reasoning_content.get("text", "")
             streaming_chunk = StreamingChunk(
                 content="",
                 index=block_idx,
-                meta={
-                    **base_meta,
-                    "reasoning_contents": [{"index": block_idx, "reasoning_content": reasoning_content}],
-                },
+                reasoning=ReasoningContent(
+                    reasoning_text=reasoning_text,
+                    extra={"reasoning_contents": [{"index": block_idx, "reasoning_content": reasoning_content}]},
+                ),
+                meta=base_meta,
             )
 
     elif "messageStop" in event:
@@ -642,7 +644,10 @@ def _process_reasoning_contents(chunks: list[StreamingChunk]) -> ReasoningConten
     reasoning_signature = None
     redacted_content = None
     for chunk in chunks:
-        reasoning_contents = chunk.meta.get("reasoning_contents", [])
+        if chunk.reasoning and chunk.reasoning.extra:
+            reasoning_contents = chunk.reasoning.extra.get("reasoning_contents", [])
+        else:
+            reasoning_contents = []
 
         for reasoning_content in reasoning_contents:
             content_block_index = reasoning_content["index"]

integrations/amazon_bedrock/tests/test_chat_generator_utils.py

@@ -1269,6 +1269,12 @@ def test_callback(chunk: StreamingChunk):
         ]
         assert replies == expected_messages
 
+        # Verify streaming chunks carry reasoning in the reasoning field, not in meta
+        reasoning_chunks = [c for c in streaming_chunks if c.reasoning is not None]
+        assert len(reasoning_chunks) > 0
+        for chunk in reasoning_chunks:
+            assert "reasoning_contents" not in chunk.meta
+
     def test_process_streaming_response_with_one_tool_call_with_redacted_thinking(self, mock_boto3_session):
         model = "arn:aws:bedrock:us-east-1::inference-profile/us.anthropic.claude-sonnet-3-7-20250219-v1:0"
         type_ = (