update python-agent to use OpenAI tool_calls format

WarpGrep v2.1 now returns standard OpenAI tool_calls in JSON
instead of Qwen3 XML format. Rewrote search.py to use the openai
SDK and parse tool_calls from the response directly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>