## What changed
- Parse the optional `retirement_at` RFC 3339 value from model upgrade metadata, treating missing, null, or malformed values as unknown.
- Expose known retirement times from `model/list` as nullable Unix timestamps in `upgradeInfo.retirementAt` and preserve them when converting app-server models back into model presets.
- Update the generated protocol schemas and app-server documentation.
## Testing
- Cover absent, null, valid, and malformed catalog values.
- Verify `model/list` serialization and app-server model conversion for known, missing, and out-of-range timestamps.
GitOrigin-RevId: 969baf9f5b012997f460fb4611ba63c963ee658c
Shijie Rao
5cc65ecb98