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/conversion/testdata/migrated_dashboards_output/v1beta1-mig-v15.mimir_rollout_debugging.v42.v0alpha1.json

@@ -743,7 +743,9 @@
             "text": "prod",
             "value": "prod"
           },
-          "datasource": "$datasource",
+          "datasource": {
+            "uid": "$datasource"
+          },
           "hide": 0,
           "includeAll": true,
           "label": "cluster",
@@ -764,7 +766,9 @@
             "text": "prod",
             "value": "prod"
           },
-          "datasource": "$datasource",
+          "datasource": {
+            "uid": "$datasource"
+          },
           "hide": 0,
           "includeAll": false,
           "label": "namespace",

apps/dashboard/pkg/migration/conversion/testdata/migrated_dashboards_output/v1beta1-mig-v15.mimir_rollout_debugging.v42.v2alpha1.json

@@ -961,8 +961,12 @@
           "hide": "dontHide",
           "refresh": "onDashboardLoad",
           "skipUrlSync": false,
+          "datasource": {
+            "type": "",
+            "uid": "$datasource"
+          },
           "query": {
-            "kind": "prometheus",
+            "kind": "",
             "spec": {
               "__legacyStringValue": "label_values(up, job)"
             }
@@ -988,8 +992,12 @@
           "hide": "dontHide",
           "refresh": "onDashboardLoad",
           "skipUrlSync": false,
+          "datasource": {
+            "type": "",
+            "uid": "$datasource"
+          },
           "query": {
-            "kind": "prometheus",
+            "kind": "",
             "spec": {
               "__legacyStringValue": "label_values(up{job=~\"$cluster\"}, instance)"
             }

apps/dashboard/pkg/migration/conversion/testdata/migrated_dashboards_output/v1beta1-mig-v15.mimir_rollout_debugging.v42.v2beta1.json

@@ -978,8 +978,11 @@
           "skipUrlSync": false,
           "query": {
             "kind": "DataQuery",
-            "group": "prometheus",
+            "group": "",
             "version": "v0",
+            "datasource": {
+              "name": "$datasource"
+            },
             "spec": {
               "__legacyStringValue": "label_values(up, job)"
             }
@@ -1007,8 +1010,11 @@
           "skipUrlSync": false,
           "query": {
             "kind": "DataQuery",
-            "group": "prometheus",
+            "group": "",
             "version": "v0",
+            "datasource": {
+              "name": "$datasource"
+            },
             "spec": {
               "__legacyStringValue": "label_values(up{job=~\"$cluster\"}, instance)"
             }