docs: clarify function tools executed event pairing

[nightcityblade]

Summary

Clarify how FunctionToolsExecutedEvent pairs function calls with their outputs and what a None output represents.

Changes

• Document that function_calls and function_call_outputs are parallel lists paired by index.
• Note that non-None outputs share the paired call's call_id.
• Document that None means no value should be sent back to the LLM, such as StopResponse or invalid tool output.
• Add a short docstring to zipped().

Testing

• uv run ruff check livekit-agents/livekit/agents/voice/events.py
• uv run ruff format --check livekit-agents/livekit/agents/voice/events.py
• uv run pytest tests/test_agent_session.py -q

Fixes #5696

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

nightcityblade seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[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 1 additional finding.