fix(@angular/build): ensure karma polyfills reporter factory returns a value

The factory function for the `reporter:angular--polyfills` Karma plugin did not return a value, which violates Karma's plugin contract. This could cause Karma to crash when multiple reporters were in use, particularly with tools that use Karma's multi-reporter functionality. This change modifies the factory to return an object with an empty `adapters` array (`{ adapters: [] }`). This satisfies Karma's plugin requirements and ensures compatibility with the multi-reporter, preventing the crash. Fixes #31039

Author: clydin

packages/angular/build/src/builders/karma/application_builder.ts

@@ -187,6 +187,9 @@ class AngularPolyfillsPlugin {
  included: true,
  watched: false,
  });
+
+ // Karma needs a return value for a factory and Karma's multi-reporter expects an `adapters` array
+ return { adapters: [] };
  }, AngularPolyfillsPlugin),
  ],
  };