frontend-plugin-api: convert ApiRef to an opaque type

Convert the ApiRef type in the new frontend system to an opaque type
with a $$type discriminator, matching the pattern used by route refs
and extension data refs. Add a builder-pattern creation overload
(createApiRef<T>().with({ id })) alongside the existing direct-config
form. Create OpaqueApiRef in frontend-internal for internal type
validation.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

packages/frontend-internal/src/apis/OpaqueApiRef.ts

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2025 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { ApiRef } from '@backstage/frontend-plugin-api';
+import { OpaqueType } from '@internal/opaque';
+
+export const OpaqueApiRef = OpaqueType.create<{
+  public: ApiRef<unknown>;
+  versions: {
+    readonly version: 'v1';
+  };
+}>({
+  type: '@backstage/ApiRef',
+  versions: ['v1'],
+});

packages/frontend-internal/src/apis/index.ts

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2025 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+export { OpaqueApiRef } from './OpaqueApiRef';

packages/frontend-internal/src/index.ts

@@ -14,5 +14,6 @@
  * limitations under the License.
  */
 
+export * from './apis';
 export * from './routing';
 export * from './wiring';