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/v42.harky_must.json

@@ -0,0 +1,31 @@
+{
+    "title": "v42 Migration Test - Harky Must",
+    "schemaVersion": 41,
+    "panels": [
+      {
+        "id": 1,
+        "title": "Panel with hideFrom.viz = true",
+        "type": "timeseries",
+        "fieldConfig": {
+          "defaults": {},
+          "overrides": [
+            {
+              "matcher": {
+                "id": "byName",
+                "options": "Field1"
+              },
+              "properties": [
+                {
+                  "id": "custom.hideFrom",
+                  "value": {
+                    "viz": true
+                  }
+                }
+              ]
+            }
+          ]
+        }
+      }
+    ]
+  }
+