fix: bound ProcPool process wait

[he-yufeng]

Summary

• add a bounded wait when ProcPool.launch_job() is waiting for a warmed process
• retry the existing launch loop when a spawn task exits without putting a process on the queue
• add a regression test for the empty-queue failure path so the coroutine fails instead of hanging forever

To verify

• uv run pytest tests/test_ipc.py::test_proc_pool_launch_job_times_out_when_spawn_never_warms_process -q
• uv run ruff check livekit-agents\livekit\agents\ipc\proc_pool.py tests\test_ipc.py
• uv run ruff format --check livekit-agents\livekit\agents\ipc\proc_pool.py tests\test_ipc.py
• uv run python -m py_compile livekit-agents\livekit\agents\ipc\proc_pool.py tests\test_ipc.py
• git diff --check

tests/test_ipc.py::test_slow_initialization timed out after 120s on my Windows machine without producing test output; I did not treat that as part of this patch because the new regression test isolates the empty warmed-process queue path without spawning worker processes.

Closes #5868

[devin-ai-integration]

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