## Why
Remote compaction's retained-message budget counted text but not images, so
image-heavy history could retain more context than the budget represented.
## What changed
- Add the opt-in `compaction_image_budget` feature to charge retained images
using the existing image size estimate.
- Keep images and their adjacent labels atomic when truncating a boundary
message, while preserving existing text, audio, metadata, and
client-authored developer-message behavior.
- Stop backfilling older messages when an image at the truncation boundary
does not fit.
## Testing
- Add unit coverage for image boundaries, annotations, audio, text-only
messages, and client-authored developer messages.
- Add an integration test covering repeated remote compaction with the feature
enabled, disabled, and left at its default.
GitOrigin-RevId: 88868bbd09d2858939dbae3f8402e1f497b4e3c6
ashwinnathan-openai
6677fd827d