Hydrate recommended plugin metadata on selection (#39143)

## What changed

- Fetch recommendations from the Codex-specific `/ps/plugins/suggested/codex` endpoint and parse its compact response shape.
- Fetch the selected plugin's details before presenting an install request, using them to verify availability and populate connector metadata.
- Skip install elicitation when the selected recommendation is no longer available, and return a retryable response when its metadata cannot be verified.

## Testing

- Cover the new recommendation route and response shape.
- Cover metadata hydration for available plugins and rejection of unavailable plugins.

GitOrigin-RevId: 2b0e2d70572aae2b7cd8e458b42e9dd14be3dbaf

codex-rs/app-server/tests/suite/v2/recommended_plugins.rs

@@ -55,7 +55,7 @@ async fn recommended_plugins_after_external_login(
     let server = responses::start_mock_server().await;
     let apps_server = AppsTestServer::mount(&server).await?;
     Mock::given(method("GET"))
-        .and(path("/ps/plugins/suggested"))
+        .and(path("/ps/plugins/suggested/codex"))
         .and(query_param("scope", "GLOBAL"))
         .respond_with(
             ResponseTemplate::new(200)
@@ -65,9 +65,7 @@ async fn recommended_plugins_after_external_login(
                     "plugins": [{
                         "id": "plugin_github",
                         "name": "github",
-                        "status": "ENABLED",
-                        "installation_policy": "AVAILABLE",
-                        "release": {"display_name": "GitHub"}
+                        "display_name": "GitHub"
                     }]
                 })),
         )