Dashboard Schema v1beta1 to v2alpha1: Preserve string template variable datasource references in query variables (#115516)

* Dashboard migration: preserve legacy string datasource references

Fix v1beta1 → v2alpha1 conversion to handle legacy string datasource
references in QueryVariable, AdhocVariable, and GroupByVariable.

Previously, string datasource references (both template variables like
"$datasource" and direct names/UIDs like "prometheus") were being
dropped during conversion, causing variable chaining to break.

The frontend's DatasourceSrv.getInstanceSettings() already handles
string references by trying uid → name → id lookup at runtime, so we
preserve the string in the uid field and let the frontend resolve it.

* trigger frontend ci tests when dashboard migration code changes

* v1: if string convert to DS ref

* Update migration testdata to fix template variable datasource references

* update

apps/dashboard/pkg/migration/testdata/input/v15.mimir_rollout_debugging.json

@@ -654,7 +654,9 @@
           "text": "prod",
           "value": "prod"
         },
-        "datasource": "$datasource",
+        "datasource": {
+          "uid": "$datasource"
+        },
         "hide": 0,
         "includeAll": true,
         "label": "cluster",
@@ -677,7 +679,9 @@
           "text": "prod",
           "value": "prod"
         },
-        "datasource": "$datasource",
+        "datasource": {
+          "uid": "$datasource"
+        },
         "hide": 0,
         "includeAll": false,
         "label": "namespace",