frontend-app-api: fix phased predicate type checks

Use internal extension shapes when reading predicate metadata and type the finalized app test helper explicitly. This fixes the typecheck breakage introduced by the phased predicate and override changes.

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

packages/frontend-internal/src/wiring/InternalFrontendPlugin.ts

@@ -17,10 +17,10 @@
 import {
   Extension,
   FeatureFlagConfig,
-  FilterPredicate,
   IconElement,
   OverridableFrontendPlugin,
 } from '@backstage/frontend-plugin-api';
+import { FilterPredicate } from '@backstage/filter-predicates';
 import { JsonObject } from '@backstage/types';
 import { OpaqueType } from '@internal/opaque';