Search selected plugin apps before falling back (#38484)

## What changed

- When an explicitly selected plugin has apps available, instruct the model to use `tool_search` to discover relevant tools before falling back to unrelated or built-in tools.
- Scope the guidance to the request that selected the plugin and omit it for plugins without available apps.

## Testing

- Cover app-specific rendering, non-app plugins, bounded instruction output, and app enablement in dual-surface plugin requests.

GitOrigin-RevId: 1b9501cc403d2817334a9307e683e99c140a794a

codex-rs/core/tests/suite/plugins.rs

@@ -1134,6 +1134,13 @@ async fn explicit_plugin_mentions_use_apps_for_chatgpt_dual_surface_plugins(
         app_enabled,
         "plugin app guidance should match app enablement: {developer_messages:?}"
     );
+    assert_eq!(
+        developer_messages
+            .iter()
+            .any(|text| text.contains("if `tool_search` is available")),
+        app_enabled,
+        "plugin app search guidance should match app enablement: {developer_messages:?}"
+    );
     assert!(
         request
             .tool_by_name(SAMPLE_PLUGIN_MCP_NAMESPACE, "echo")