Move ExtensionFactoryMiddleware from frontend-plugin-api to frontend-app-api

The `ExtensionFactoryMiddleware` type is only used by the app-level wiring
and has been moved to `@backstage/frontend-app-api` as its canonical location.

The type is preserved in `@backstage/frontend-plugin-api` with a `@deprecated`
tag pointing to the new location. All internal usages have been updated to
import from the new location.

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

packages/frontend-app-api/report.api.md

@@ -4,10 +4,13 @@
 
 ```ts
 import { ApiHolder } from '@backstage/core-plugin-api';
+import { ApiHolder as ApiHolder_2 } from '@backstage/frontend-plugin-api';
 import { AppNode } from '@backstage/frontend-plugin-api';
 import { AppTree } from '@backstage/frontend-plugin-api';
 import { ConfigApi } from '@backstage/core-plugin-api';
-import { ExtensionFactoryMiddleware } from '@backstage/frontend-plugin-api';
+import { ExtensionDataContainer } from '@backstage/frontend-plugin-api';
+import { ExtensionDataRef } from '@backstage/frontend-plugin-api';
+import { ExtensionDataValue } from '@backstage/frontend-plugin-api';
 import { ExternalRouteRef } from '@backstage/frontend-plugin-api';
 import { FrontendFeature } from '@backstage/frontend-plugin-api';
 import { FrontendPlugin } from '@backstage/frontend-plugin-api';
@@ -177,6 +180,18 @@ export type CreateSpecializedAppOptions = {
   };
 };
 
+// @public (undocumented)
+export type ExtensionFactoryMiddleware = (
+  originalFactory: (contextOverrides?: {
+    config?: JsonObject;
+  }) => ExtensionDataContainer<ExtensionDataRef>,
+  context: {
+    node: AppNode;
+    apis: ApiHolder_2;
+    config?: JsonObject;
+  },
+) => Iterable<ExtensionDataValue<any, any>>;
+
 // @public
 export type FrontendPluginInfoResolver = (ctx: {
   packageJson(): Promise<JsonObject | undefined>;

packages/frontend-app-api/src/tree/instantiateAppNodeTree.test.ts

@@ -19,7 +19,6 @@ import {
   Extension,
   ExtensionDataRef,
   ExtensionDefinition,
-  ExtensionFactoryMiddleware,
   ExtensionInput,
   PortableSchema,
   ResolvedExtensionInput,
@@ -29,6 +28,7 @@ import {
   createExtensionInput,
   createFrontendPlugin,
 } from '@backstage/frontend-plugin-api';
+import { ExtensionFactoryMiddleware } from '../wiring/types';
 import {
   createAppNodeInstance,
   instantiateAppNodeTree,

packages/frontend-app-api/src/tree/instantiateAppNodeTree.ts

@@ -18,10 +18,10 @@ import {
   ApiHolder,
   ExtensionDataContainer,
   ExtensionDataRef,
-  ExtensionFactoryMiddleware,
   ExtensionInput,
   ResolvedExtensionInputs,
 } from '@backstage/frontend-plugin-api';
+import { ExtensionFactoryMiddleware } from '../wiring/types';
 import mapValues from 'lodash/mapValues';
 import { AppNode, AppNodeInstance } from '@backstage/frontend-plugin-api';
 // eslint-disable-next-line @backstage/no-relative-monorepo-imports

packages/frontend-app-api/src/wiring/createSpecializedApp.tsx

@@ -29,9 +29,9 @@ import {
   createApiFactory,
   routeResolutionApiRef,
   AppNode,
-  ExtensionFactoryMiddleware,
   FrontendFeature,
 } from '@backstage/frontend-plugin-api';
+import { ExtensionFactoryMiddleware } from './types';
 import {
   AnyApiFactory,
   ApiHolder,

packages/frontend-app-api/src/wiring/index.ts

@@ -20,3 +20,4 @@ export {
 } from './createSpecializedApp';
 export { type FrontendPluginInfoResolver } from './createPluginInfoAttacher';
 export { type AppError, type AppErrorTypes } from './createErrorCollector';
+export { type ExtensionFactoryMiddleware } from './types';

packages/frontend-app-api/src/wiring/types.ts

@@ -0,0 +1,36 @@
+/*
+ * 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 { JsonObject } from '@backstage/types';
+import {
+  ApiHolder,
+  AppNode,
+  ExtensionDataContainer,
+  ExtensionDataRef,
+  ExtensionDataValue,
+} from '@backstage/frontend-plugin-api';
+
+/** @public */
+export type ExtensionFactoryMiddleware = (
+  originalFactory: (contextOverrides?: {
+    config?: JsonObject;
+  }) => ExtensionDataContainer<ExtensionDataRef>,
+  context: {
+    node: AppNode;
+    apis: ApiHolder;
+    config?: JsonObject;
+  },
+) => Iterable<ExtensionDataValue<any, any>>;