feat: load plugin apps (#13401)

load plugin-apps from `.app.json`.

make apps runtime-mentionable iff `codex_apps` MCP actually exposes
tools for that `connector_id`.

if the app isn't available, it's filtered out of runtime connector set,
so no tools are added and no app-mentions resolve.

right now we don't have a clean cli-side error for an app not being
installed. can look at this after.

### Tests
Added tests, tested locally that using a plugin that bundles an app
picks up the app.

codex-rs/core/src/plugins/mod.rs

@@ -2,6 +2,7 @@ mod manager;
 mod manifest;
 mod store;
 
+pub use manager::AppConnectorId;
 pub use manager::LoadedPlugin;
 pub use manager::PluginInstallError;
 pub use manager::PluginLoadOutcome;