[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/core-plugins/src/remote.rs

@@ -167,9 +167,29 @@ pub struct RemotePluginDetail {
     pub app_manifest: Option<JsonValue>,
     pub skills: Vec<RemotePluginSkill>,
     pub app_ids: Vec<String>,
+    pub app_templates: Vec<RemoteAppTemplate>,
     pub mcp_servers: Vec<String>,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct RemoteAppTemplate {
+    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<RemoteAppTemplateUnavailableReason>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Deserialize, Serialize)]
+#[serde(rename_all = "SCREAMING_SNAKE_CASE")]
+pub enum RemoteAppTemplateUnavailableReason {
+    NotConfiguredForWorkspace,
+    NoActiveWorkspace,
+}
+
 #[derive(Debug, Clone, PartialEq)]
 pub struct RemotePluginSkill {
     pub name: String,
@@ -413,6 +433,8 @@ struct RemotePluginReleaseResponse {
     app_ids: Vec<String>,
     #[serde(default)]
     app_manifest: Option<JsonValue>,
+    #[serde(default, alias = "unavailable_app_templates")]
+    app_templates: Vec<RemoteAppTemplateResponse>,
     #[serde(default)]
     keywords: Vec<String>,
     interface: RemotePluginReleaseInterfaceResponse,
@@ -427,6 +449,24 @@ struct RemotePluginMcpServerResponse {
     key: String,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq, Deserialize, Serialize)]
+struct RemoteAppTemplateResponse {
+    template_id: String,
+    name: String,
+    #[serde(default)]
+    description: Option<String>,
+    #[serde(default)]
+    canonical_connector_id: Option<String>,
+    #[serde(default)]
+    logo_url: Option<String>,
+    #[serde(default)]
+    logo_url_dark: Option<String>,
+    #[serde(default)]
+    materialized_app_ids: Vec<String>,
+    #[serde(default)]
+    reason: Option<RemoteAppTemplateUnavailableReason>,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, Deserialize, Serialize)]
 struct RemotePluginDirectoryItem {
     id: String,
@@ -975,6 +1015,21 @@ async fn build_remote_plugin_detail(
         app_manifest: plugin.release.app_manifest,
         skills,
         app_ids: plugin.release.app_ids,
+        app_templates: plugin
+            .release
+            .app_templates
+            .into_iter()
+            .map(|template| RemoteAppTemplate {
+                template_id: template.template_id,
+                name: template.name,
+                description: template.description,
+                canonical_connector_id: template.canonical_connector_id,
+                logo_url: template.logo_url,
+                logo_url_dark: template.logo_url_dark,
+                materialized_app_ids: template.materialized_app_ids,
+                reason: template.reason,
+            })
+            .collect(),
         mcp_servers,
     })
 }