TUI Plugin Sharing 2 - add remote plugin section plumbing (#26702)

This adds the background plumbing for remote-backed plugin catalog
sections while leaving the fuller directory presentation to the next PR.
The TUI can fetch section-specific remote marketplace results, keep
local plugin data available, and carry section errors forward for later
rendering.

- Fetches explicit remote marketplace kinds for curated, workspace, and
shared-with-me sections.
- Gates shared-with-me loading on the plugin sharing feature flag.
- Adds section-level error state and user-actionable error copy.
- Merges remote marketplace results into the cached plugin list without
discarding local results.

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

@@ -644,6 +644,12 @@ impl PluginRequestProcessor {
                     data.push(remote_marketplace_to_info(remote_marketplace));
                 }
                 Ok(None) => {}
+                Err(err) if explicit_marketplace_kinds => {
+                    return Err(remote_plugin_catalog_error_to_jsonrpc(
+                        err,
+                        "list OpenAI Curated remote plugin catalog",
+                    ));
+                }
                 Err(
                     RemotePluginCatalogError::AuthRequired
                     | RemotePluginCatalogError::UnsupportedAuthMode,

codex-rs/app-server/tests/suite/v2/plugin_list.rs

@@ -2064,7 +2064,7 @@ async fn plugin_list_includes_openai_curated_remote_collection_when_requested()
 }
 
 #[tokio::test]
-async fn plugin_list_fail_opens_openai_curated_remote_collection_errors() -> Result<()> {
+async fn plugin_list_propagates_explicit_openai_curated_remote_collection_errors() -> Result<()> {
     let codex_home = TempDir::new()?;
     let server = MockServer::start().await;
     write_plugins_enabled_config_with_base_url(
@@ -2103,18 +2103,17 @@ async fn plugin_list_fail_opens_openai_curated_remote_collection_errors() -> Res
             marketplace_kinds: Some(vec![PluginListMarketplaceKind::Vertical]),
         })
         .await?;
-    let response: JSONRPCResponse = timeout(
+    let err = timeout(
         DEFAULT_TIMEOUT,
-        mcp.read_stream_until_response_message(RequestId::Integer(request_id)),
+        mcp.read_stream_until_error_message(RequestId::Integer(request_id)),
     )
     .await??;
-    let response: PluginListResponse = to_response(response)?;
 
+    assert_eq!(err.error.code, -32603);
     assert!(
-        response
-            .marketplaces
-            .iter()
-            .all(|marketplace| marketplace.name != "openai-curated-remote")
+        err.error
+            .message
+            .contains("list OpenAI Curated remote plugin catalog")
     );
     Ok(())
 }