* 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
37c1e3fb02