Add a minimum file size guard (MIN_AUDIO_FILE_BYTES = 1024) before
sending audio to transcription APIs. Files below this threshold are
almost certainly empty or corrupt and would cause unhelpful errors
from Whisper/Deepgram/Groq providers.
Changes:
- Add 'tooSmall' skip reason to MediaUnderstandingSkipError
- Add MIN_AUDIO_FILE_BYTES constant (1024 bytes) to defaults
- Guard both provider and CLI audio paths in runner.ts
- Add comprehensive tests for tiny, empty, and valid audio files
- Update existing test fixtures to use audio files above threshold
