GitHub Copilot's API returns tool_calls split across multiple choices:
- choices[0]: content only (tool_calls=null)
- choices[1]: tool_calls only (content=null)
The existing _parse_response only inspected choices[0], so tool_calls
were silently lost, causing the agent to never execute tools when using
github_copilot/ models.
This fix scans all choices and merges tool_calls + content, so
providers that return multi-choice responses work correctly.
Single-choice providers (OpenAI, Anthropic, etc.) are unaffected since
the loop over one choice is equivalent to the original code.
