fix(openai): avoid duplicate responses function calls

[nightcityblade]

Fixes #5136

Summary

• when using previous_response_id, filter pending function_call items from the delta context while preserving their function_call_output items
• add a regression test for Responses API context reuse

Tests

• uv run ruff check livekit-plugins/livekit-plugins-openai/livekit/plugins/openai/responses/llm.py tests/test_openai_responses_llm.py
• uv run ruff format --check livekit-plugins/livekit-plugins-openai/livekit/plugins/openai/responses/llm.py tests/test_openai_responses_llm.py
• uv run pytest tests/test_openai_responses_llm.py

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[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.