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,