Dashboard Controls: Fix schema transformation for link placement (#114630)

* fix: add missing transformation for scenes -> save model v2

* fix: link placement transformation on the backend between schemas

* fix: update the openapi spec in the tests

* tes: add tests for `transformSceneToSaveModelSchemaV2`

* tests: extend conversion_test.go to cover link placements

apps/dashboard/pkg/migration/conversion/testdata/output/v1beta1.dashboard-properties.v2beta1.json

@@ -137,7 +137,8 @@
         "asDropdown": false,
         "targetBlank": true,
         "includeVars": false,
-        "keepTime": false
+        "keepTime": false,
+        "placement": "inControlsMenu"
       },
       {
         "title": "Tag-based Link",