One sniffer owns magic-byte container detection (Teknium's one-concept-
one-owner rule): the new tools/audio_container.py is used by
- gateway/platforms/base.py _sniff_audio_ext (inbound cache — PR #36166's
central sniffer, now covering AAC/ADTS, MP4-brand disambiguation, webm)
- gateway/platforms/signal.py _guess_extension (audio/AV branches
delegated; RIFF/WAVE fix from PR #50690 and M4A-brand fix from
PR #72490 now live centrally)
- tools/tts_tool.py _sniff_audio_container (outbound repair, PR #73072)
cache_audio_from_url inherits the sniff via cache_audio_from_bytes.
Adds tests/tools/test_audio_container.py covering every magic-byte type,
wrong-extension repair on the inbound cache, unknown passthrough, the
URL path, and Signal's delegation.
