## What changed
- Add protocol types and `ResponseItem` helpers for attaching and clearing
locally recorded tool-call names and arguments.
- Bound serialized metadata across a prompt, replacing oversized arguments and
omitted calls with trusted truncation details.
- Prevent deserialized response items or model-provided arguments from forging
locally generated tool-call and truncation metadata.
## Testing
- Cover prompt-wide size limits, omission accounting, idempotent bounding, and
forged truncation markers.
GitOrigin-RevId: 960a223437ce81fe6b4cbb5fc7a9774408b2755b
