fix: handle mid-stream sample rate/format changes in decode_audio

[g0rdonL]

Bug

decode_audio crashes on audio files whose stream changes sample rate or sample format partway through. _group_frames writes every decoded frame into a single AudioFifo, but AudioFifo locks onto the format/layout/rate of the first frame it receives and raises ValueError on any later frame that differs:

  File "faster_whisper/audio.py", line 96, in _group_frames
      fifo.write(frame)
    File "av/audio/fifo.pyx", line 81, in av.audio.fifo.AudioFifo.write
  ValueError: Frame does not match AudioFifo parameters.

AudioResampler has the same locking behavior and can hit the equivalent error in _resample_frames once frames from different source-parameter groups reach it (this surfaces after fixing the FIFO issue alone).

Fix

Catch the ValueError in both _group_frames and _resample_frames, flush what's already buffered, and start a fresh FIFO/resampler for the frame with the new parameters. The final AudioResampler step still normalizes everything to the requested output format/layout/rate, so decoded output is unaffected in the common (single sample-rate) case.

Verification

• Reproduced the crash on macOS arm64 with the exact sample file (whisper-french-mixed-samplerate.mp3) attached to decode_audio crashes with "Frame does not match AudioFifo parameters" on files whose sample rate/format changes mid-stream #1451 — the file's stream switches 44100 Hz → 48000 Hz partway through (confirmed via ffprobe).
• After the fix, decode_audio succeeds on that file (17.08s of mono 16kHz float32 audio).
• Confirmed decode_audio output is byte-identical before/after this change on all 5 existing fixtures in tests/data/ (jfk.flac, multilingual.mp3, physicsworks.wav, stereo_diarization.wav, hotwords.mp3), including split_stereo=True — this fix is a no-op on the happy path.
• Added tests/test_audio.py with a regression test using the reporter's sample file (added as tests/data/mixed_samplerate.mp3).
• Full suite: pytest tests/ → 20 passed (19 existing + 1 new).
• black, isort, flake8 all clean per CONTRIBUTING.md.

Fixes #1451

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Drafted and verified with AI assistance (repro, fix, and test run locally); reviewed before submission.