Fix WebRTC callback shutdown deadlock

[voropaevv]

Why

WebRTCSession.close() may be called by on_data and on_error callbacks on the session event-loop thread. It previously submitted cleanup coroutines to that same loop and blocked on Future.result(), preventing the loop from running them. The public state changed to CLOSED before teardown, so a later close() call could not recover the pending cleanup.

Fixes #2777

What changed

• dispatch event-loop initiated cleanup to one daemon helper without blocking the loop
• serialize cleanup ownership so peer, source, and loop resources are torn down once
• keep external and context-manager shutdown synchronous by waiting for cleanup completion
• preserve source and loop cleanup when peer shutdown fails, and allow retry if the helper thread cannot start
• add deterministic real-loop lifecycle tests for callback-first, startup-lock, external-first, context-manager, duplicate-close, and failure schedules

Verification

• baseline regression test fails on the unmodified parent because the event-loop callback never returns
• .venv/bin/python -m pytest tests/inference_sdk/unit_tests/webrtc/test_session_lifecycle.py -q — 32 passed
• .venv/bin/python -m pytest tests/inference_sdk/unit_tests/webrtc -q — 83 passed
• .venv/bin/python -m pytest tests/inference_sdk/unit_tests -q — 519 passed, 86 existing deprecation warnings
• lifecycle coverage run exercises every executable line added to the shutdown implementation
• PATH="$PWD/.venv/bin:$PATH" make check_code_quality — passed
• git diff --check — passed

Not verified

• no live remote WebRTC server or browser session was used; the regression is thread/loop lifecycle logic covered with a real background asyncio loop and async fake peer/source objects
• cleanup of a peer connection constructed before _init() assigns self._pc is separate pre-existing startup-failure behavior and remains outside this change

Risks / rollback

The change is isolated to the SDK close lifecycle. External close() remains synchronous; only calls originating on the internal event loop are dispatched. Reverting commit 7da042d9e08d30a367e505e89ba91a6429fad55f restores the previous behavior.

Migrations / external effects

None.

Screenshots / preview

Not applicable; no UI changes.