fix(rtc): avoid deprecated asyncio.iscoroutinefunction in EventEmitter.on

[xbt-a4224j]

Fixes #790

Problem

EventEmitter.on() calls asyncio.iscoroutinefunction() to reject coroutine callbacks. That function is deprecated in Python 3.14 and slated for removal in 3.16 (CPython gh-122858), so on 3.14 every callback registration emits a DeprecationWarning. Room and friends register many handlers, so this is noisy in practice — the existing test suite alone produced 14 of them.

Fix

inspect.iscoroutinefunction() is the documented replacement, but it is not a drop-in: asyncio.iscoroutinefunction additionally treats any callable tagged with the private sentinel asyncio.coroutines._is_coroutine as a coroutine function. On Python 3.9, inspect.iscoroutinefunction(AsyncMock()) returns False — a bare swap would let a mocked async callback past the guard, so emit() would call the mock and silently discard the unawaited coroutine.

The fix keeps the sentinel check alongside inspect.iscoroutinefunction, read with getattr(..., None) so it degrades cleanly if the private attribute is removed in 3.16. Behaviour is identical to asyncio.iscoroutinefunction on every supported version and the deprecated function is never called.

Testing

Four tests added to tests/rtc/test_emitter.py, all running on every version in the matrix:

• test_on_does_not_warn — promotes DeprecationWarning to an error around registration.
• test_on_rejects_async_callback — async def and functools.partial-wrapped.
• test_on_rejects_sentinel_tagged_callback — exercises the sentinel branch directly.
• test_on_rejects_async_mock — the real-world case; on 3.9 only the sentinel catches it.

Verified as a real flip against the released livekit wheel on all six supported Pythons (3.9–3.14): unpatched 1 failed / 7 passed / 14 warnings → patched 8 passed / 2 warnings.

[CLAassistant]

All committers have signed the CLA.

[devin-ai-integration]

✅ Devin Review: No Issues Found

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

[cloudwebrtc]

lgtm