fix: check for actual error content, not just "error" key presence
OpenAI Responses API returns {"error": null} on success, which
incorrectly triggered is_error=True. Now we check if error_data
is truthy rather than just checking key existence.
🤖 Generated with [Letta Code](https://letta.com)
Co-authored-by: Letta <noreply@letta.com>