Allow disabling plugin discovery prefetch at app-server startup (#57249)

## Why

Hosts that only need installed plugin inventory should be able to skip startup requests for plugin discovery.

## What changed

Add the stable, default-enabled `features.plugin_discovery_prefetch` flag. Setting it to `false` skips startup prefetching of the plugin directory, featured plugins, and recommendations while preserving installed plugin cache refreshes and bundle downloads.

## Testing

Update app-server integration coverage to verify that installed plugin bundles download with discovery prefetch disabled and that no directory, featured, or suggested plugin requests are made during startup. Also cover listing remote marketplaces with the flag disabled.

GitOrigin-RevId: 7cc3072f6045131b9f1acb851f6240db75ac173e

codex-rs/features/src/lib.rs

@@ -351,6 +351,9 @@ pub enum Feature {
     ComputerUse,
     /// Enable the PS-backed remote plugin catalog.
     RemotePlugin,
+    /// Prefetch the plugin directory, featured plugins, and recommendations at app-server startup.
+    /// Inventory-only hosts can disable this. Legacy suggestions read the cached directory.
+    PluginDiscoveryPrefetch,
     /// Enable remote plugin sharing flows.
     PluginSharing,
     /// Removed compatibility flag retained as a no-op.
@@ -1816,6 +1819,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::PluginDiscoveryPrefetch,
+        key: "plugin_discovery_prefetch",
+        stage: Stage::Stable,
+        default_enabled: true,
+    },
     FeatureSpec {
         id: Feature::PluginSharing,
         key: "plugin_sharing",