OpenAPI: Fix ValidationError message attribute (#90846)

It's `message`, not `msg`. It's been fixed for a while here: https://github.com/grafana/grafana-openapi-client-go/blob/9ef69836127d783a63b2d0c61b2f8bf3de0c8e86/scripts/pull-schema.sh#L46-L49

I've also reflected the change in the tests

pkg/services/ngalert/api/tooling/definitions/shared.go

@@ -11,7 +11,7 @@ type Ack struct{}
 // swagger:model
 type ValidationError struct {
 	// example: error message
-	Msg string `json:"msg"`
+	Message string `json:"message"`
 }
 
 // swagger:model