test(plugins): speed up integration tests (~45% improvement) (#5137)

* test(plugins): speed up integration tests with shared wazero cache

Reduce plugin test suite runtime from ~22s to ~12s by:

- Creating a shared wazero compilation cache directory in TestPlugins()
  and setting conf.Server.CacheFolder globally so all test Manager
  instances reuse compiled WASM binaries from disk cache
- Moving 6 createTestManager* calls from inside It blocks to BeforeAll
  blocks in scrobbler_adapter_test.go and manager_call_test.go
- Replacing time.Sleep(2s) in KVStore TTL test with Eventually polling
- Reducing WebSocket callback sleeps from 100ms to 10ms

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

* test(plugins): enhance websocket tests by storing server messages for verification

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

---------

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

plugins/testdata/test-websocket/main.go

@@ -45,10 +45,11 @@ func (t *testWebSocket) OnTextMessage(input websocket.OnTextMessageRequest) erro
 }
 
 // OnBinaryMessage is called when a binary message is received.
+// Echoes the data back as a text message prefixed with "binary_echo:" so tests
+// can observe the callback fired.
 func (t *testWebSocket) OnBinaryMessage(input websocket.OnBinaryMessageRequest) error {
-	// Store received binary data for test verification
 	storeReceivedMessage("binary:" + input.Data)
-	return nil
+	return host.WebSocketSendText(input.ConnectionID, "binary_echo:"+input.Data)
 }
 
 // OnError is called when an error occurs on a WebSocket connection.