fix: add empty choices guard in extract_openai_tool_calls() (#12540)

## Summary

`extract_openai_tool_calls()` in `llm.py` crashes with `IndexError` when
the LLM provider returns a response with an empty `choices` list.

### Changes 🏗️

- Added a guard check `if not response.choices: return None` before
accessing `response.choices[0]`
- This is consistent with the function's existing pattern of returning
`None` when no tool calls are found

### Bug Details

When an LLM provider returns a response with an empty choices list
(e.g., due to content filtering, rate limiting, or API errors),
`response.choices[0]` raises `IndexError`. This can crash the entire
agent execution pipeline.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- Verified that the function returns `None` when `response.choices` is
empty
- Verified existing behavior is unchanged when `response.choices` is
non-empty

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Co-authored-by: goingforstudying-ctrl <[email protected]>
Co-authored-by: Claude Opus 4.6 <[email protected]>
Co-authored-by: Zamil Majdy <[email protected]>