fix(openai): delete trimmed empty audio items

[he-yufeng]

Summary

• delete items explicitly trimmed out by update_chat_ctx(), even if the local copy has empty content
• keep the empty-content guard for updates so unsynced audio/image items are not recreated
• add a unit test for empty audio-like remote items being deleted from the OpenAI realtime context diff

Fixes #5886.

To verify

• uv run pytest tests/test_realtime/test_openai_realtime_model.py -q
• uv run ruff check livekit-plugins\livekit-plugins-openai\livekit\plugins\openai\realtime\realtime_model.py tests\test_realtime\test_openai_realtime_model.py
• uv run ruff format --check livekit-plugins\livekit-plugins-openai\livekit\plugins\openai\realtime\realtime_model.py tests\test_realtime\test_openai_realtime_model.py
• uv run python -m py_compile livekit-plugins\livekit-plugins-openai\livekit\plugins\openai\realtime\realtime_model.py tests\test_realtime\test_openai_realtime_model.py
• git diff --check

Notes

• uv run python scripts\check_types.py currently fails on Windows in livekit-agents\livekit\agents\cli\readchar.py because mypy sees POSIX termios/tty attributes as missing. The errors are outside this diff.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.

[theomonnom]

Seems like https://github.com/he-yufeng/agents/blob/29ad338bcf2ce014f1f6c62f9cadc166a1f7e422/livekit-plugins/livekit-plugins-openai/livekit/plugins/openai/realtime/realtime_model.py#L1360 is now unused ?

[he-yufeng]

Good catch. I kept that helper for the update path below: when an existing item id is recreated with different content, it still avoids recreating server-only audio/image items whose content was never synced down. This PR only changes the remove path so those trimmed empty items can be deleted from the server context.