fix(openai): skip streaming chunks without choices

[cyyecao-lappland]

Issue Link (REQUIRED)

Fixes #706

Summary

Changes

Skip OpenAI streaming chunks with an empty choices list before indexing the first choice. Add regression tests using actual SDK chunk objects for usage-only chunks at the start, middle, end, and alone, and for propagation of genuine stream errors. Update the method docstring.

User experience

A usage-only chunk currently raises IndexError and prevents delivery of the accumulated final message. After the fix, text callbacks and the final message are preserved; usage chunks do not generate empty token events.

Validation

• Four usage-only cases failed on main; all five new cases pass with the fix.
• OpenAI-related tests: 12 passed.
• Full Python suite: 442 passed, 1 failure. Main without this fix has 437 passed and the identical failure in test_database_connection_handling: libsql cannot open the Windows temporary database path. No database or storage code is changed.
• Full production-source Ruff lint passes; new-test lint and formatting pass. The repository's full formatting check reports 47 existing source files requiring formatting, so this PR keeps the source diff focused instead of reformatting unrelated code.
• Tested on Windows with Python 3.11; all model responses are mocked locally.

Checklist

• I have performed a self-review of this change (automated code inspection; no human review is claimed).
• Changes have been tested.
• Changes are documented.
• I have linked this PR to an existing issue (required).

This is a bug fix with no public API changes or new dependencies.

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

AI disclosure

This contribution was autonomously authored and tested with Codex (GPT-6), at the account owner's explicit request. Tests use local mocked model responses; no live model call or human review is claimed.