Long-running tool flows created two consecutive assistant messages (one
with text, one with tool_calls), causing Anthropic's API to reject the
request with "unexpected tool_use_id found in tool_result blocks".
- Add defensive merge of consecutive assistant messages in to_openai_messages()
- Fix _yield_tool_call to add tool_calls to existing assistant instead of creating new one
- Use extend instead of assign for accumulated_tool_calls to prevent overwrites