Test: HTTP-Attrappe um headers ergänzt

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/test_core_contract.py

@@ -501,7 +501,7 @@ def test_http_clients_follow_redirects_and_3xx_is_never_success(monkeypatch):
     seen = []
 
     class R:
-        status_code, content, text = 200, b"[]", "[]"
+        status_code, content, text, headers = 200, b"[]", "[]", {}
         def json(self): return []
 
     monkeypatch.setattr(httpx, "request", lambda *a, **k: (seen.append(k), R())[1])