feat: align google and relay providers

pkg/wsrelay/message.go

@@ -0,0 +1,19 @@
+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     = "http_request"
+	MessageTypeHTTPResp    = "http_response"
+	MessageTypeStreamStart = "stream_start"
+	MessageTypeStreamChunk = "stream_chunk"
+	MessageTypeStreamEnd   = "stream_end"
+	MessageTypeError       = "error"
+	MessageTypePing        = "ping"
+	MessageTypePong        = "pong"
+)