## Why
Serde's generic buffering for flattened and internally tagged fields is incompatible with `serde_json`'s `arbitrary_precision` representation. As a result, persisted rollout items containing nested floating-point values can fail to decode.
## What changed
- Decode the rollout envelope at the JSON persistence boundary before deserializing the item payload.
- Use the shared decoder for both session resume and thread-history projection.
- Cover token-count floats, response-item metadata, field ordering, and ignored metadata in the decoder compatibility test.
GitOrigin-RevId: 59c7f6fbcf83162b5ed6980a69381fe45fc20a0a