fix(tests): unblock AudioProcessingTrack recv() silence tests (#1022)

## Summary

- CI test runs were timing out at ~2h (recent runs cancelled at 1h43m /
1h56m / 2h0m). Root cause: PR #1001 ("Delay A/V in WebRTC until both
tracks have been seen.") changed `AudioProcessingTrack.recv()` so the
first call now blocks in `_wait_for_initial_audio()` until enough audio
has been buffered, but three tests in
`tests/test_audio_processing_track.py` still constructed tracks with
`started=False` and fed no / insufficient / paused audio — so `recv()`
looped forever.
- Drop `started=False` from the silence-fallback tests
(`test_no_audio_returns_silence`, `test_paused_returns_silence`,
`test_undersized_chunk_returns_silence`) so they exercise the
post-anchor `recv()` path, which is the path these silence-fallback
assertions actually care about.
- Local `pytest` now completes in ~70s instead of hanging.

## Test plan

- [x] `uv run pytest tests/test_audio_processing_track.py -v` — 30/30
pass in ~1.2s
- [x] `uv run pytest` — full suite completes in ~70s (vs. previously
hanging on `test_no_audio_returns_silence`)
- [x] `uv run ruff check tests/test_audio_processing_track.py` clean
- [x] `uv run ruff format --check tests/test_audio_processing_track.py`
clean

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: Rafał Leszko <rafal@livepeer.org>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: leszko

tests/test_audio_processing_track.py

@@ -200,7 +200,7 @@ def test_preserves_dc(self):
 
 class TestRecv:
     def test_no_audio_returns_silence(self):
-        track = _make_track(started=False)
+        track = _make_track()
         frame = _run(track.recv())
         raw = np.frombuffer(bytes(frame.planes[0]), dtype=np.int16)
         assert np.all(raw == 0)
@@ -247,7 +247,7 @@ def test_1d_tensor_treated_as_mono(self):
         assert isinstance(frame, AudioFrame)
 
     def test_paused_returns_silence(self):
-        track = _make_track(started=False)
+        track = _make_track()
         track.frame_processor.paused = True
         audio = torch.randn(2, SAMPLES_PER_FRAME + 100)
         track.frame_processor.get_audio_packet = MagicMock(
@@ -259,7 +259,7 @@ def test_paused_returns_silence(self):
         assert np.all(raw == 0)
 
     def test_undersized_chunk_returns_silence(self):
-        track = _make_track(started=False)
+        track = _make_track()
         audio = torch.randn(2, 100)  # too small for a 960-sample frame
         track.frame_processor.get_audio_packet = MagicMock(
             side_effect=_once(audio, 48000)