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/adapter_websocket_callback.go

@@ -9,7 +9,7 @@ import (
 )
 
 // newWasmWebSocketCallback creates a new adapter for a WebSocketCallback plugin
-func newWasmWebSocketCallback(wasmPath, pluginID string, m *Manager, runtime api.WazeroNewRuntime, mc wazero.ModuleConfig) WasmPlugin {
+func newWasmWebSocketCallback(wasmPath, pluginID string, m *managerImpl, runtime api.WazeroNewRuntime, mc wazero.ModuleConfig) WasmPlugin {
 	loader, err := api.NewWebSocketCallbackPlugin(context.Background(), api.WazeroRuntime(runtime), api.WazeroModuleConfig(mc))
 	if err != nil {
 		log.Error("Error creating WebSocket callback plugin", "plugin", pluginID, "path", wasmPath, err)