Fix dashboard migration discrepancies between backend and frontend implementations (use toEqual) (#110268)

**Highlights**

* **Single-version migrations**: add `targetVersion` to migrator & model, separate outputs, enforce exact version.
* **Datasource fixes**: include `apiVersion` in tests, empty-string → `{}`, preserve `{}` refs, drop unwanted defaults.
* **Panel defaults & nesting**: only top-level panels get defaults; preserve empty `transformations` context-aware; filter repeated panels.

* **Migration parity**

  * V16: collapsed rows, grid height parsing (`px`).
  * V17: omit `maxPerRow` when `minSpan=1`.
  * V19–V20: cleanup defaults (`targetBlank`, style).
  * V23–V24: template vars + table panel consistency.
  * V28: full singlestat/stat parity, mappings & color.
  * V30–V36: threshold logic, empty refs, nested targets.
* **Save-model cleanup**: replicate frontend defaults/filtering, drop null IDs, metadata, unused props.
* **Testing**: unified suites, dev dashboards (v42), full unit coverage for major migrations.

Co-authored-by: Ivan Ortega [ivanortegaalba@gmail.com](mailto:ivanortegaalba@gmail.com)
Co-authored-by: Dominik Prokop [dominik.prokop@grafana.com](mailto:dominik.prokop@grafana.com)

pkg/tests/apis/dashboard/dashboards_test.go

@@ -47,7 +47,7 @@ func runDashboardTest(t *testing.T, helper *apis.K8sTestHelper, gvr schema.Group
 			Object: map[string]interface{}{
 				"spec": map[string]any{
 					"title":         "Test empty dashboard",
-					"schemaVersion": 41,
+					"schemaVersion": 42,
 				},
 			},
 		}

pkg/tests/apis/dashboard/testdata/dashboard-test-v1.yaml

@@ -6,4 +6,4 @@ spec:
   title: Test dashboard. Created at v1
   uid: test-v1 # will be removed by mutation hook
   version: 1234567 # will be removed by mutation hook
-  schemaVersion: 41
+  schemaVersion: 42

pkg/tests/apis/preferences/stars_test.go

@@ -52,7 +52,7 @@ func TestIntegrationStars(t *testing.T) {
 					},
 					"spec": map[string]any{
 						"title":         fmt.Sprintf("test %d", i),
-						"schemaVersion": 41, // not really!
+						"schemaVersion": 42, // not really!
 						"panels":        []any{},
 					},
 				},