Add Parakeet TDT VAD chunking

[0xShug0]

Summary

• add request-level audio_chunk_mode=vad support for Parakeet TDT offline ASR
• use Silero VAD to skip silent spans before Parakeet decoding
• document the new VAD mode and add a CLI path test case
• preserve existing auto, fixed, and none behavior

Resolves #314

Validation

• built audiocpp_cli
• ran existing Parakeet offline and streaming path tests
• ran the new Parakeet offline VAD path test
• checked direct CLI runs for audio_chunk_mode=vad, fixed, and none