API: return resource ID when deleting datasource with UID and library element (#41342)

* API: return resource ID when deleting datasource and library element

* keep status codes consistent

* fix element ID

* add test

* improve response struct

* update endpoints documentation

* LibraryElementIDResponse -> DeleteLibraryElementResponse

pkg/services/libraryelements/models.go

@@ -207,3 +207,9 @@ type LibraryElementArrayResponse struct {
 type LibraryElementConnectionsResponse struct {
 	Result []LibraryElementConnectionDTO `json:"result"`
 }
+
+// DeleteLibraryElementResponse is the response struct for deleting a library element.
+type DeleteLibraryElementResponse struct {
+	ID      int64  `json:"id"`
+	Message string `json:"message"`
+}