Expose model upgrade retirement times (#38449)

## 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

codex-rs/app-server/src/models.rs

@@ -34,6 +34,10 @@ fn model_from_preset(preset: ModelPreset) -> Model {
             upgrade_copy: upgrade.upgrade_copy.clone(),
             model_link: upgrade.model_link.clone(),
             migration_markdown: upgrade.migration_markdown.clone(),
+            retirement_at: upgrade
+                .retirement_at
+                .as_ref()
+                .map(chrono::DateTime::timestamp),
         }),
         availability_nux: preset.availability_nux.map(Into::into),
         display_name: preset.display_name.to_string(),