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.

This commit updates the redirect response creation in AngularServerApp to merge the custom headers from responseInit.headers into the redirect response headers, ensuring that cookies and other custom headers are preserved.

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 () => {