feat: add websocket routing and executor unregister API

- Introduce Server.AttachWebsocketRoute(path, handler) to mount websocket
  upgrade handlers on the Gin engine.
- Track registered WS paths via wsRoutes with wsRouteMu to prevent
  duplicate registrations; initialize in NewServer and import sync.
- Add Manager.UnregisterExecutor(provider) for clean executor lifecycle
  management.
- Add github.com/gorilla/websocket v1.5.3 dependency and update go.sum.

Motivation: enable services to expose WS endpoints through the core server
and allow removing auth executors dynamically while avoiding duplicate
route setup. No breaking changes.

internal/wsrelay/message.go

@@ -0,0 +1,27 @@
+package wsrelay
+
+// Message represents the JSON payload exchanged with websocket clients.
+type Message struct {
+	ID      string         `json:"id"`
+	Type    string         `json:"type"`
+	Payload map[string]any `json:"payload,omitempty"`
+}
+
+const (
+	// MessageTypeHTTPReq identifies an HTTP-style request envelope.
+	MessageTypeHTTPReq = "http_request"
+	// MessageTypeHTTPResp identifies a non-streaming HTTP response envelope.
+	MessageTypeHTTPResp = "http_response"
+	// MessageTypeStreamStart marks the beginning of a streaming response.
+	MessageTypeStreamStart = "stream_start"
+	// MessageTypeStreamChunk carries a streaming response chunk.
+	MessageTypeStreamChunk = "stream_chunk"
+	// MessageTypeStreamEnd marks the completion of a streaming response.
+	MessageTypeStreamEnd = "stream_end"
+	// MessageTypeError carries an error response.
+	MessageTypeError = "error"
+	// MessageTypePing represents ping messages from clients.
+	MessageTypePing = "ping"
+	// MessageTypePong represents pong responses back to clients.
+	MessageTypePong = "pong"
+)