Preload plugin recommendations during session startup (#41375)

## Why

Fetching plugin recommendations while building the first-turn context delays that
context when the recommendation catalog is not already cached.

## What changed

- Start fetching recommendations during session initialization, in parallel with
  MCP and plugin setup, and reuse the in-flight request when building context.
- Centralize the feature checks that enable plugin recommendations.
- Prevent an invalidated recommendation request from repopulating the cache after
  a newer request has completed.

## Testing

Added coverage for startup feature gating, reuse of the in-flight fetch, legacy
behavior when recommendations are unavailable, and cache invalidation races.

GitOrigin-RevId: 708774b9bfc472c9847973ae8b2774b66e7321e4

codex-rs/core/src/session/mod.rs

@@ -3731,11 +3731,10 @@ impl Session {
             .plugins_manager
             .plugins_for_config(&turn_context.config.plugins_config_input())
             .await;
-        let features = turn_context.config.features.get();
-        let recommended_plugin_candidates = if features.enabled(Feature::Apps)
-            && features.enabled(Feature::Plugins)
-            && (features.enabled(Feature::ToolSuggest)
-                || features.enabled(Feature::RecommendedPlugins))
+        let recommended_plugin_candidates = if turn_context
+            .config
+            .features
+            .plugin_recommendations_enabled()
         {
             let auth = self.services.auth_manager.auth().await;
             let plugins_config = turn_context.config.plugins_config_input();