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/dev-dashboards-output/panel-table/table_sparkline_cell.v42.json

@@ -183,6 +183,24 @@
       },
       "fieldConfig": {
         "defaults": {
+          "actions": [
+            {
+              "fetch": {
+                "body": "{}",
+                "headers": [
+                  [
+                    "Content-Type",
+                    "application/json"
+                  ]
+                ],
+                "method": "GET",
+                "queryParams": [],
+                "url": "/api/health"
+              },
+              "title": "Get instance health",
+              "type": "fetch"
+            }
+          ],
           "color": {
             "fixedColor": "blue",
             "mode": "shades"
@@ -195,6 +213,13 @@
             },
             "inspect": false
           },
+          "links": [
+            {
+              "targetBlank": true,
+              "title": "Google Grafana",
+              "url": "https://google.com/search?q=grafana"
+            }
+          ],
           "mappings": [],
           "thresholds": {
             "mode": "absolute",
@@ -208,27 +233,7 @@
                 "value": 80
               }
             ]
-          },
-          "actions": [
-            {
-              "fetch": {
-                "body": "{}",
-                "headers": [["Content-Type", "application/json"]],
-                "method": "GET",
-                "queryParams": [],
-                "url": "/api/health"
-              },
-              "title": "Get instance health",
-              "type": "fetch"
-            }
-          ],
-          "links": [
-            {
-              "targetBlank": true,
-              "title": "Google Grafana",
-              "url": "https://google.com/search?q=grafana"
-            }
-          ]
+          }
         },
         "overrides": []
       },
@@ -612,4 +617,4 @@
   "title": "Panel Tests - Table - Sparklines",
   "uid": "d6373b49-1957-4f00-9218-ee2120d3ecd9",
   "weekStart": ""
-}
+}