test(audio): add edge cases for pad_audio single waveform, chunk boundaries, and base64 limit

[nwalio]

Seven additional tests for audio.py covering paths the existing suite misses:

• pad_audio with a single waveform (no padding needed)
• pad_audio with identical-length waveforms (no zero-fill)
• pad_audio with multiple > 1 rounds width up correctly
• chunk_audio with chunk_size == len(samples) yields one chunk
• chunk_audio with chunk_size > len(samples) yields one short chunk
• pcm_encode / pcm_decode roundtrip at boundary values (-1, +1)
• audio_from_base64 with a payload exactly at the byte limit

All 44 tests pass locally (pytest tests/test_audio.py -q).