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/v37.go

@@ -115,10 +115,10 @@ func processPanelsV37(panels []interface{}) {
 			continue
 		}
 
-		displayMode, _ := legend["displayMode"].(string)
+		displayMode := GetStringValue(legend, "displayMode")
 		showLegend, hasShowLegend := legend["showLegend"].(bool)
 
-		// If displayMode is "hidden" OR showLegend is false, normalize to hidden legend
+		// If displayMode is "hidden" OR showLegend is explicitly false, normalize to hidden legend
 		if displayMode == "hidden" || (hasShowLegend && !showLegend) {
 			legend["displayMode"] = "list"
 			legend["showLegend"] = false