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/v29.go

@@ -44,16 +44,12 @@ func V29(_ context.Context, dashboard map[string]interface{}) error {
 			continue
 		}
 		// Set refresh to 1 if not 1 or 2
-		refresh, hasRefresh := variable["refresh"]
-		refreshInt := 0
-		if r, ok := refresh.(int); ok {
-			refreshInt = r
-		}
-		if !hasRefresh || (refreshInt != 1 && refreshInt != 2) {
+		refreshInt := GetIntValue(variable, "refresh", 0)
+		if refreshInt != 1 && refreshInt != 2 {
 			variable["refresh"] = 1
 		}
-		// Clear options if present
-		if _, hasOptions := variable["options"]; hasOptions {
+		// Clear options if they have content (matches frontend behavior)
+		if options, hasOptions := variable["options"].([]interface{}); hasOptions && len(options) > 0 {
 			variable["options"] = []interface{}{}
 		}
 	}