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)

apps/dashboard/pkg/migration/schemaversion/v30_test.go

@@ -25,7 +25,7 @@ func TestV30(t *testing.T) {
 									"steps": []interface{}{
 										map[string]interface{}{
 											"color": "green",
-											"value": nil,
+											"value": (*float64)(nil),
 										},
 										map[string]interface{}{
 											"color": "red",
@@ -99,7 +99,7 @@ func TestV30(t *testing.T) {
 									"steps": []interface{}{
 										map[string]interface{}{
 											"color": "green",
-											"value": nil,
+											"value": (*float64)(nil),
 										},
 										map[string]interface{}{
 											"color": "red",
@@ -112,12 +112,10 @@ func TestV30(t *testing.T) {
 										"type": "value",
 										"options": map[string]interface{}{
 											"1": map[string]interface{}{
-												"text":  "Up",
-												"color": "green",
+												"text": "Up",
 											},
 											"0": map[string]interface{}{
-												"text":  "Down",
-												"color": "green",
+												"text": "Down",
 											},
 										},
 									},
@@ -127,8 +125,7 @@ func TestV30(t *testing.T) {
 											"from": float64(10),
 											"to":   float64(20),
 											"result": map[string]interface{}{
-												"text":  "Medium",
-												"color": "green",
+												"text": "Medium",
 											},
 										},
 									},
@@ -137,8 +134,7 @@ func TestV30(t *testing.T) {
 										"options": map[string]interface{}{
 											"match": "null",
 											"result": map[string]interface{}{
-												"text":  "Null Value",
-												"color": "green",
+												"text": "Null Value",
 											},
 										},
 									},