fix(@angular/ssr): preserve response headers during redirect

Previously, when a redirect was triggered, any custom headers (such as cookies or cache control) configured on the RESPONSE_INIT object by components or guards were lost because the redirect response was created with the default headers only.

Closes angular#33473

Author: alan-agius4

packages/angular/ssr/src/app.ts

@@ -353,7 +353,11 @@ export class AngularServerApp {
     }
 
     if (result.redirectTo) {
-      return createRedirectResponse(result.redirectTo, responseInit.status, headers);
+      return createRedirectResponse(
+        result.redirectTo,
+        responseInit.status,
+        Object.fromEntries(responseInit.headers),
+      );
     }
 
     if (renderMode === RenderMode.Prerender) {

packages/angular/ssr/test/app_spec.ts

@@ -42,6 +42,9 @@ describe('AngularServerApp', () => {
         const responseInit = inject(RESPONSE_INIT);
         if (responseInit) {
           responseInit.status = 308;
+          if (responseInit.headers && typeof (responseInit.headers as Headers).set === 'function') {
+            (responseInit.headers as Headers).set('X-Redirect-Header', 'custom-value');
+          }
         }
 
         void inject(Router).navigate([], {
@@ -326,6 +329,7 @@ describe('AngularServerApp', () => {
         const response = await app.handle(new Request('http://localhost/redirect-via-navigate'));
         expect(response?.headers.get('location')).toBe('/redirect-via-navigate?filter=test');
         expect(response?.status).toBe(308);
+        expect(response?.headers.get('X-Redirect-Header')).toBe('custom-value');
       });
 
       it('returns a 302 status and redirects to the correct location when `urlTree` is updated in a guard', async () => {