Migrate from pytest-style to unittest.IsolatedAsyncioTestCase to match the
pattern used by other transport tests (e.g. test_livekit_transport.py). Guard
the aiortc/av import with try/except and skipUnless so tests gracefully skip
when webrtc dependencies aren't installed. Add pyright suppressions for false
positives inherent to testing internals of optional-dependency classes.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>