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/testdata/input/v6.pulldowns_and_templating.json

@@ -12,14 +12,20 @@
       "annotations": [
         {
           "name": "deployment",
-          "datasource": "prometheus",
+          "datasource": {
+            "uid": "default-ds-uid",
+            "type": "prometheus"
+          },
           "enable": true,
           "iconColor": "red",
           "query": "ALERTS{alertname=\"DeploymentStarted\"}"
         },
         {
           "name": "alerts",
-          "datasource": "loki",
+          "datasource": {
+            "uid": "loki-uid",
+            "type": "loki"
+          },
           "enable": false,
           "iconColor": "yellow",
           "query": "{job=\"alertmanager\"}"
@@ -32,12 +38,14 @@
       {
         "name": "environment",
         "type": "filter",
-        "allFormat": null
+        "allFormat": null,
+        "query": "label_values(up, instance)"
       },
       {
         "name": "service",
         "datasource": "prometheus",
-        "allFormat": "glob"
+        "allFormat": "glob",
+        "query": "label_values(up, instance)"
       },
       {
         "name": "region",