## Why
Audio variants were available in the user-input protocol but were replaced with unsupported-input placeholders instead of being sent to the model.
## What changed
- Serialize audio data URLs as `input_audio` content and convert local `wav`, `mp3`, `m4a`, `webm`, and `ogg` files to labeled data URLs.
- Validate and canonicalize base64 audio before requests, enforce the 50 MiB input limit, and replace invalid, unsupported, or oversized audio with explanatory text.
- Preserve audio attachments when mapping response items back to user-message events and document the app-server input variants.
## Testing
- Add unit coverage for local-file conversion, data URL validation, size and format errors, event mapping, and attachment extraction.
- Add client tests that verify data URL and local audio payloads sent to the Responses API.
GitOrigin-RevId: f72cd6b6e43ab12757eeb47621b1d670594ac7d9