Stop prefetching plugin discovery at app-server startup (#59768)

## What changed

- Remove startup prefetching of remote catalogs, featured plugins, and recommendations while retaining installed-plugin refreshes and bundle downloads.
- Fetch featured plugin IDs on demand, concurrently with marketplace fetching when listing the remote global catalog, and reuse the cached results.
- Retire `plugin_discovery_prefetch` as an ignored compatibility flag and remove forced background catalog refreshes.

## Testing

Update app-server tests to verify that startup downloads installed bundles without discovery requests and that two `plugin/list` calls share one featured-plugin fetch.

GitOrigin-RevId: aa10474f3429102d624c83472e81ec85e3b470ae

codex-rs/app-server/src/message_processor.rs

@@ -592,7 +592,6 @@ impl MessageProcessor {
             turn_cost_worker.as_ref().map(TurnCostWorker::handle),
         );
         if let Some(startup_config) = plugin_startup_tasks {
-            // Keep plugin startup warmups aligned at app-server startup.
             let reload_config = match startup_config {
                 PluginStartupConfig::Current => {
                     plugin_config_reload::for_cwd(config_manager.clone(), config.cwd.clone())
@@ -607,7 +606,6 @@ impl MessageProcessor {
                 .plugins_manager()
                 .maybe_start_plugin_startup_tasks_for_config(
                     &config.plugins_config_input(),
-                    config.features.enabled(Feature::PluginDiscoveryPrefetch),
                     reload_config,
                     Some(on_effective_plugins_changed),
                     startup_warnings_tx,