Add CAPI support for MAI in voice mode and dictation

[meganrogge]

Add CAPI support for MAI so it can be used by both voice mode and dictation.

Scope

• Enable MAI through CAPI for voice mode.
• Enable MAI through CAPI for dictation.
• Ensure both experiences use the appropriate CAPI request/response flow and handle errors consistently with existing providers.
• Add coverage for provider selection, successful transcription, and failure/cancellation paths.

Done when

• Voice mode can use MAI through CAPI end to end.
• Dictation can use MAI through CAPI end to end.
• Existing voice mode and dictation providers continue to work unchanged.
• Relevant automated tests and documentation are updated.

[techwhizbang]

Voice-Code-Transcription-API.pdf