Relax remote plugin sync gate (#22594)

## Summary
- Allow remote installed-plugin cache refresh to start whenever plugins
are enabled.
- Allow remote installed-plugin bundle sync to start whenever plugins
are enabled.
- Remove the extra local `remote_plugin_enabled` guard from those
background sync paths.

## Context
Server-side installed plugin state and optional bundle URL behavior are
owned by plugin-service `/public/plugins/installed`, so these local sync
paths only need the overall plugin enablement gate.

## Test plan
- `just fmt`
- `cargo test -p codex-core-plugins`

codex-rs/app-server/src/request_processors/plugins.rs

@@ -450,14 +450,15 @@ impl PluginRequestProcessor {
             return Ok(empty_response());
         }
         let plugins_input = config.plugins_config_input();
-        let (mut data, marketplace_load_errors) = if include_local {
+        if include_local || marketplace_kinds.contains(&PluginListMarketplaceKind::SharedWithMe) {
             plugins_manager.maybe_start_plugin_list_background_tasks_for_config(
                 &plugins_input,
                 auth.clone(),
                 &roots,
                 Some(self.effective_plugins_changed_callback()),
             );
-
+        }
+        let (mut data, marketplace_load_errors) = if include_local {
             let config_for_marketplace_listing = plugins_input.clone();
             let plugins_manager_for_marketplace_listing = plugins_manager.clone();
             let shared_plugin_ids_by_local_path = load_shared_plugin_ids_by_local_path(&config)?;
@@ -916,6 +917,9 @@ impl PluginRequestProcessor {
         params: PluginShareUpdateTargetsParams,
     ) -> Result<PluginShareUpdateTargetsResponse, JSONRPCErrorError> {
         let (config, auth) = self.load_plugin_share_config_and_auth().await?;
+        if !config.features.enabled(Feature::PluginSharing) {
+            return Err(invalid_request("plugin sharing is disabled"));
+        }
         let PluginShareUpdateTargetsParams {
             remote_plugin_id,
             discoverability,