chore: plugin/uninstall endpoint (#14111)

add `plugin/uninstall` app-server endpoint to fully rm plugin from
plugins cache dir and rm entry from user config file.

plugin-enablement is session-scoped, so uninstalls are only picked up in
new sessions (like installs).

added tests.

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

@@ -17,6 +17,7 @@ pub use manager::PluginCapabilitySummary;
 pub use manager::PluginInstallError;
 pub use manager::PluginInstallRequest;
 pub use manager::PluginLoadOutcome;
+pub use manager::PluginUninstallError;
 pub use manager::PluginsManager;
 pub use manager::load_plugin_apps;
 pub(crate) use manager::plugin_namespace_for_skill_path;