refactor: modularize openai responses schema

[ashwinb]

Summary

• replace the monolithic openai_responses module with a package that re-exports focused submodules for messages, tool calls, tooling, streaming objects, and other response structures
• update the generator to group upstream definitions into those submodules, emit module-specific imports, and clean the target package before writing fresh output
• drop the manual subset snapshot now that the generator produces grouped modules directly

Testing

• python scripts/generate_openai_responses.py --output /tmp/openai_responses_gen
• python -m compileall llama_stack/apis/agents/openai_responses

---

https://chatgpt.com/codex/tasks/task_e_68e94c246b7c832180ce4df999720cd0