fix(@angular/ssr): ensure unique values in redirect response Vary header

Refactors the `createRedirectResponse` function to use a `Set` for constructing the `Vary` header. This ensures that `X-Forwarded-Prefix` is always present exactly once, and that existing `Vary` values from provided headers are correctly parsed, deduplicated, and preserved.

Updates the associated unit tests to reflect the new header order and verify the deduplication logic.

(cherry picked from commit ff1160e)

Author: alan-agius4

packages/angular/ssr/src/utils/redirect.ts

@@ -50,13 +50,18 @@ export function createRedirectResponse(
     );
   }
 
-  let vary = resHeaders.get('Vary') ?? '';
-  if (vary) {
-    vary += ', ';
+  // Ensure unique values for Vary header
+  const varyArray = resHeaders.get('Vary')?.split(',') ?? [];
+  const varySet = new Set(['X-Forwarded-Prefix']);
+  for (const vary of varyArray) {
+    const value = vary.trim();
+
+    if (value) {
+      varySet.add(value);
+    }
   }
-  vary += 'X-Forwarded-Prefix';
 
-  resHeaders.set('Vary', vary);
+  resHeaders.set('Vary', [...varySet].join(', '));
   resHeaders.set('Location', location);
 
   return new Response(null, {

packages/angular/ssr/test/utils/redirect_spec.ts

@@ -36,7 +36,14 @@ describe('Redirect Utils', () => {
         'Vary': 'Host',
       });
       expect(response.headers.get('Location')).toBe('/home');
-      expect(response.headers.get('Vary')).toBe('Host, X-Forwarded-Prefix');
+      expect(response.headers.get('Vary')).toBe('X-Forwarded-Prefix, Host');
+    });
+
+    it('should NOT add duplicate X-Forwarded-Prefix if already present in Vary header', () => {
+      const response = createRedirectResponse('/home', 302, {
+        'Vary': 'X-Forwarded-Prefix, Host',
+      });
+      expect(response.headers.get('Vary')).toBe('X-Forwarded-Prefix, Host');
     });
 
     it('should warn if Location header is provided in extra headers in dev mode', () => {