[codex] Expose unavailable app templates in plugin detail (#26317)

## Summary
- Adds `unavailable_app_templates` to the app-server protocol and
generated schemas/types.
- Parses plugin-service `release.unavailable_app_templates` in the
remote plugin client.
- Maps remote unavailable templates into app-server `PluginDetail`.
- Defaults local plugins to an empty unavailable app template list.

## Validation
- `just write-app-server-schema`
- `cargo +1.95.0 fmt --manifest-path codex-rs/Cargo.toml --all --check`
- `cargo +1.95.0 test --manifest-path codex-rs/Cargo.toml -p
codex-app-server-protocol schema_fixtures`
- `cargo +1.95.0 check --manifest-path codex-rs/Cargo.toml -p
codex-app-server-protocol -p codex-core-plugins -p codex-app-server`
- `git diff --check`

Note: default `cargo check` uses rustc 1.89 locally and failed because
dependencies require newer Rust, so validation was rerun with installed
Rust 1.95.

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

@@ -642,9 +642,32 @@ pub struct PluginDetail {
     pub skills: Vec<SkillSummary>,
     pub hooks: Vec<PluginHookSummary>,
     pub apps: Vec<AppSummary>,
+    pub app_templates: Vec<AppTemplateSummary>,
     pub mcp_servers: Vec<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "SCREAMING_SNAKE_CASE")]
+#[ts(export_to = "v2/")]
+pub enum AppTemplateUnavailableReason {
+    NotConfiguredForWorkspace,
+    NoActiveWorkspace,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct AppTemplateSummary {
+    pub template_id: String,
+    pub name: String,
+    pub description: Option<String>,
+    pub canonical_connector_id: Option<String>,
+    pub logo_url: Option<String>,
+    pub logo_url_dark: Option<String>,
+    pub materialized_app_ids: Vec<String>,
+    pub reason: Option<AppTemplateUnavailableReason>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]