fix(audio): map webm and mp4 container mime types to audio extensions

[mrmorgan-i]

Description

finfo reports browser MediaRecorder output as video/webm, since WebM is a container format. That isn't in GeneratesAudioFilename's match list, so it falls through to default => 'mp3' and the file gets uploaded as audio.mp3 with WebM bytes inside it. OpenAI fails to parse it with:

OpenAI Error [400]: invalid_request_error - Audio file might be corrupted or unsupported

Same thing happens with video/mp4, which is what Safari's MediaRecorder produces.

This adds the container types to the existing match list. The default => 'mp3' fallback is unchanged, along with the three tests covering it.

Related to #981. That issue covers the audio/x-wav alias, and #1001 handles it at the Media layer, which makes sense for aliases that are for sure audio. video/webm is different because it's a truthful mime type, and normalizing it in Media would break genuine video input such as Gemini's. So this only touches the audio filename helper, and the two changes shouldn't conflict.