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/v18.go

@@ -47,10 +47,10 @@ import "context"
 func V18(_ context.Context, dashboard map[string]interface{}) error {
 	dashboard["schemaVersion"] = 18
 
-	panels, ok := dashboard["panels"].([]interface{})
-	if !ok {
+	if !IsArray(dashboard["panels"]) {
 		return nil
 	}
+	panels := dashboard["panels"].([]interface{})
 
 	for _, p := range panels {
 		panel, ok := p.(map[string]interface{})
@@ -91,12 +91,15 @@ func migrateGaugePanelOptions(panel map[string]interface{}) {
 
 	options["valueOptions"] = valueOptions
 
-	if thresholds, ok := optionsGauge["thresholds"].([]interface{}); ok && len(thresholds) > 0 {
-		reversedThresholds := make([]interface{}, len(thresholds))
-		for i, threshold := range thresholds {
-			reversedThresholds[len(thresholds)-1-i] = threshold
+	if IsArray(optionsGauge["thresholds"]) {
+		thresholds := optionsGauge["thresholds"].([]interface{})
+		if len(thresholds) > 0 {
+			reversedThresholds := make([]interface{}, len(thresholds))
+			for i, threshold := range thresholds {
+				reversedThresholds[len(thresholds)-1-i] = threshold
+			}
+			options["thresholds"] = reversedThresholds
 		}
-		options["thresholds"] = reversedThresholds
 	}
 
 	// Copy any other properties from options-gauge to options