fix(plugins): silence plugin warnings and folder creation when plugins disabled (#4297)

* fix(plugins): silence repeated “Plugin not found” spam for inactive Spotify/Last.fm plugins

Navidrome was emitting a warning when the optional Spotify or
Last.fm agents weren’t enabled, filling the journal with entries like:

    level=warning msg="Plugin not found" capability=MetadataAgent name=spotify

Fixed by completely disable the plugin system when Plugins.Enabled = false.

Signed-off-by: Deluan <deluan@navidrome.org>

* style: update test description for clarity

Signed-off-by: Deluan <deluan@navidrome.org>

* fix: ensure plugin folder is created only if plugins are enabled

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

plugins/host_websocket.go

@@ -50,12 +50,12 @@ func (s WebSocketHostFunctions) Close(ctx context.Context, req *websocket.CloseR
 // websocketService implements the WebSocket service functionality
 type websocketService struct {
 	connections map[string]*WebSocketConnection
-	manager     *Manager
+	manager     *managerImpl
 	mu          sync.RWMutex
 }
 
 // newWebsocketService creates a new websocketService instance
-func newWebsocketService(manager *Manager) *websocketService {
+func newWebsocketService(manager *managerImpl) *websocketService {
 	return &websocketService{
 		connections: make(map[string]*WebSocketConnection),
 		manager:     manager,