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/v17.go

@@ -82,7 +82,8 @@ func migrateMinSpanToMaxPerRow(panel map[string]interface{}) {
 	max := gridColumnCount / minSpan
 	factors := getFactors(gridColumnCount)
 
-	// Find the first factor greater than max
+	// Find the first factor greater than max, then use the previous factor
+	// This matches the frontend logic: findIndex(factors, (o) => o > max) - 1
 	factorIndex := -1
 	for i, factor := range factors {
 		if float64(factor) > max {
@@ -91,16 +92,20 @@ func migrateMinSpanToMaxPerRow(panel map[string]interface{}) {
 		}
 	}
 
-	// Use the previous factor as maxPerRow
+	// Use the previous factor as maxPerRow (matching frontend logic exactly)
+	// The frontend code does: factors[findIndex(factors, (o) => o > max) - 1]
+	// When findIndex returns -1, this becomes factors[-2] which is undefined
+	// So we need to match this behavior
 	if factorIndex > 0 {
 		panel["maxPerRow"] = factors[factorIndex-1]
 	} else if factorIndex == 0 {
 		// If the first factor is already greater than max, use 1
 		panel["maxPerRow"] = 1
-	} else {
-		// If no factor is greater than max, use the largest factor
-		panel["maxPerRow"] = factors[len(factors)-1]
 	}
+	// If no factor is greater than max, don't set maxPerRow
+	// This matches frontend behavior when findIndex returns -1
+	// The frontend sets maxPerRow to undefined, which gets filtered out
+	// So we don't set it at all
 
 	// Remove the minSpan property
 	delete(panel, "minSpan")