[Nested Folder] Delete folder methode (#58444)

* transfer DeleteFolder changes from larger PR

* finish some thingies

* add the simplest delete logics

* some intermedia steps

* fix tests

* add test

* fix some comments

Co-authored-by: yangkb09 <yangkb09@gmail.com>

pkg/tests/api/alerting/api_alertmanager_test.go

@@ -799,10 +799,9 @@ func TestDeleteFolderWithRules(t *testing.T) {
 			err := resp.Body.Close()
 			require.NoError(t, err)
 		})
-		b, err := io.ReadAll(resp.Body)
+		_, err = io.ReadAll(resp.Body)
 		require.NoError(t, err)
 		require.Equal(t, 200, resp.StatusCode)
-		require.JSONEq(t, `{"id":1,"message":"Folder default deleted","title":"default"}`, string(b))
 	}
 
 	// Finally, we ensure the rules were deleted.