## What changed
- Add `audio` to the model input modalities exposed by the protocol and generated app-server schemas.
- Preserve audio in prompts for models that advertise audio input, and replace historical audio with an omission marker for models that do not.
## Testing
- Cover prompt normalization with and without audio support.
- Extend the model-switching integration test to verify that switching from a multimodal model to a text-only model strips prior image and audio content.
GitOrigin-RevId: c18e47ad602720216303ff9abb5bca4d508391ee