* fix: normalize audio MIME types in STT format validation
Use getFileExtensionFromMime() to normalize non-standard MIME types
(e.g. audio/x-m4a, audio/x-wav, audio/x-flac) before checking against
the accepted formats list in azureOpenAIProvider. This is the same class
of bug as #12608 (text/x-markdown), but for STT audio validation.
Only audio/ and video/ MIME prefixes are normalized to prevent
non-audio types from matching via the webm default fallback.
Export getFileExtensionFromMime for testability.
Fixes#12632
* fix: reject unknown audio subtypes in STT format validation
Use MIME_TO_EXTENSION_MAP for normalization instead of
getFileExtensionFromMime() which falls back to 'webm' for unrecognized
types. Gate raw subtype matching on audio/video prefix to prevent
non-audio types (e.g. text/webm) from passing validation.
Resolves Codex review comment about unknown subtypes silently passing.
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Co-authored-by: Tobias Jonas <t.jonas@innfactory.de>
