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-protocol/src/protocol/v2/model.rs

@@ -128,6 +128,9 @@ pub struct ModelUpgradeInfo {
     pub upgrade_copy: Option<String>,
     pub model_link: Option<String>,
     pub migration_markdown: Option<String>,
+    /// Informational Unix timestamp for this upgrade's scheduled retirement, if known.
+    #[ts(type = "number | null")]
+    pub retirement_at: Option<i64>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]