Expose plugin installation timestamps in app-server summaries (#35859)

## What changed

- Add nullable `installedAt` metadata to `PluginSummary` and its generated schemas, expressed as a Unix timestamp in seconds.
- Preserve the backend installation time for remote plugins across plugin list, installed, read, and share-list responses. Return `null` when the timestamp is unavailable, including for local or uninstalled plugins and older backend responses.

## Testing

- Cover timestamp conversion in the remote plugin list integration test and the field's serialization and backward-compatible default in protocol tests.

GitOrigin-RevId: 8a1640320adb298874e1074e9891d1017ad06937

codex-rs/app-server-protocol/src/protocol/v2/plugin.rs

@@ -648,6 +648,10 @@ pub struct PluginSummary {
     pub share_context: Option<PluginShareContext>,
     pub source: PluginSource,
     pub installed: bool,
+    /// Unix timestamp in seconds when the remote plugin was installed, when available.
+    #[serde(default)]
+    #[ts(type = "number | null")]
+    pub installed_at: Option<i64>,
     pub enabled: bool,
     pub install_policy: PluginInstallPolicy,
     pub install_policy_source: Option<PluginInstallPolicySource>,