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/migration_utils.go

@@ -1,5 +1,10 @@
 package schemaversion
 
+import (
+	"strconv"
+	"strings"
+)
+
 // migration_utils.go contains shared utility functions used across multiple schema version migrations.
 
 // GetStringValue safely extracts a string value from a map, returning empty string if not found or not a string
@@ -55,6 +60,14 @@ func ConvertToFloat(value interface{}) (float64, bool) {
 		return float64(v), true
 	case int32:
 		return float64(v), true
+	case string:
+		// Handle string values like "700px" - strip px suffix and parse
+		// This matches frontend behavior: parseInt(height.replace('px', ''), 10)
+		cleanStr := strings.TrimSuffix(v, "px")
+		if parsed, err := strconv.ParseFloat(cleanStr, 64); err == nil {
+			return parsed, true
+		}
+		return 0, false
 	default:
 		return 0, false
 	}
@@ -77,3 +90,12 @@ func ConvertToInt(value interface{}) (int, bool) {
 		return 0, false
 	}
 }
+
+// IsArray checks if a value is an array (slice)
+func IsArray(value interface{}) bool {
+	if value == nil {
+		return false
+	}
+	_, ok := value.([]interface{})
+	return ok
+}