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/app-server/tests/suite/v2/mod.rs

@@ -19,6 +19,7 @@ mod output_schema;
 mod plan_item;
 mod plugin_install;
 mod plugin_list;
+mod plugin_uninstall;
 mod rate_limits;
 mod realtime_conversation;
 mod request_permissions;