fix: use plugin service route for remote uninstall

[ericning-o]

Why

Remote plugin uninstall requests were sent to /plugins/{plugin_id}/uninstall, but the plugin service expects these mutations under /ps/plugins.

What changed

• Send remote uninstall requests to /ps/plugins/{plugin_id}/uninstall.
• Update app-server uninstall tests and request assertions to use the corrected route.

Testing

• just test -p codex-core-plugins (203 passed)
• Verified against production using the locally built app-server: the uninstall succeeded, and a follow-up plugin list reported installed: false and enabled: false.