Add end-to-end tests for audio/video publish & subscribe

[cloudwebrtc]

Summary

• Adds two new end-to-end tests under livekit-rtc/tests/ that exercise the full publish → SFU → subscribe path for both audio and video tracks:
  • test_audio.py: publishes a 5-tone sine sweep (100/300/500/700/1000 Hz @ 48 kHz mono) and verifies each received tone via FFT peak detection + band-energy dominance (tolerates 15 Hz Opus jitter).
  • test_video.py: publishes a 640×480 @ 15 fps RGBA color cycle (red/green/blue/white/black, 1 s each) and verifies the subscriber observes the colors in order using nearest-palette classification over stable runs.
• Both tests auto-skip when LIVEKIT_URL / LIVEKIT_API_KEY / LIVEKIT_API_SECRET are not set, so local and CI runs without credentials stay green.
• Received media is also persisted next to the tests (subscriber_recv_freqs.wav, subscriber_recv_frame_color_*.png) to make failures easy to debug.

Motivation

We didn't have automated coverage that validated the actual media payload after it round-trips through a real SFU — unit tests only cover local-side plumbing. These tests give us a regression signal for encoder/transport/decoder behavior in the Python SDK.

Test plan

• LIVEKIT_URL=... LIVEKIT_API_KEY=... LIVEKIT_API_SECRET=... pytest livekit-rtc/tests/test_audio.py -v
• LIVEKIT_URL=... LIVEKIT_API_KEY=... LIVEKIT_API_SECRET=... pytest livekit-rtc/tests/test_video.py -v
• Confirm the tests are skipped (not failed) when the env vars are absent.