fix(ipc): mark supervised processes dead for prometheus

[Ricky-7-Yan]

Summary

• restore the Prometheus multiprocess teardown hook that was added in Configure Prometheus in multi process mode #3565 and later lost during the 1.3.0 process rewrite
• call mark_process_dead(pid) after the supervised child is joined and before its Process handle is closed
• keep teardown fail-open and add regression coverage to the existing supervised-process lifecycle test

This intentionally does not implement wildcard deletion of every *_<pid>.db file; that broader policy is separate from prometheus_client's documented dead-process hook.

Fixes #7006.

Testing

• focused supervised-process teardown pytest: 1 passed
• ruff check livekit-agents/livekit/agents/ipc/supervised_proc.py tests/test_supervised_proc_memory.py
• ruff format --check livekit-agents/livekit/agents/ipc/supervised_proc.py tests/test_supervised_proc_memory.py
• python -m py_compile livekit-agents/livekit/agents/ipc/supervised_proc.py tests/test_supervised_proc_memory.py
• git diff --check

The focused test used the current source and test files against the released livekit-agents==1.7.1 dependency set because GitHub's filtered blob transport was intermittently resetting on this Windows host.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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