Schema: convert dashboards from v1beta1 to v2beta1 (#109037)

- Implement full conversion pipeline from v1beta1 → v2beta1
- Ensure frontend–backend parity for all dashboard serialization paths
- Add automatic data loss detection for conversions (panels, queries, annotations, links, variables)
- Extract atomic conversion functions for v0 → v1beta1 → v2alpha1 → v2beta1
- Introduce conversion metrics and detailed logging for loss tracking
- Normalize datasource resolution, defaults, and annotation processing
- Improve panel layout serialization and y-coordinate normalization
- Fix inconsistencies in nested panels and collapsed row behavior
- Refine variable handling:
  - Filter refId from variable query specs
  - Default variable refresh to 'never' (matches frontend)
  - Fix constant and interval variable handling for missing queries
- Unify schema defaults (enable, hide, iconColor, editable, liveNow)
- Fix pluginId usage (UID vs type) and datasource references
- Fix metrics.go bug swallowing errors (return nil → return err)
- Add tests for version-specific conversion error handling
- Add data loss detection tests using source/target version comparison
- Clean up lint issues, legacy code, and redundant files
- Update OpenAPI snapshots and migrated dashboards
- Improve backend migrator to reuse datasource provider and match frontend logic

Co-authored-by: Haris Rozajac <haris.rozajac12@gmail.com>
Co-authored-by: Oscar Kilhed <oscar.kilhed@grafana.com>
Co-authored-by: Stephanie Hingtgen <stephanie.hingtgen@grafana.com>

apps/dashboard/pkg/migration/conversion/testdata/output/v2alpha1.complete.v2beta1.json

@@ -79,7 +79,7 @@
                   "spec": {
                     "query": {
                       "kind": "DataQuery",
-                      "group": "prometheus",
+                      "group": "",
                       "version": "v0",
                       "datasource": {
                         "name": "gdev-prometheus"
@@ -399,7 +399,7 @@
           "auto": true,
           "auto_min": "10s",
           "auto_count": 30,
-          "refresh": "load",
+          "refresh": "onTimeRangeChanged",
           "label": "Interval",
           "hide": "dontHide",
           "skipUrlSync": false,