Function plugin_unload 

pub async fn plugin_unload(
    _cfm: SharedConfium,
    _params: Value,
) -> Result<Value, RpcError>

plugin_unload({ "name": "botan" }) → {"success": true}

The core unload path is not yet implemented (the FFI is a stub), so this handler returns the engine’s “not implemented” error. When the core lands, the handler becomes a one-line adapter.