test(proxy): use a non-public test master key in the KDF thread test

check_no_publicly_known_master_key (new on main) bans the sk-1234 literal;
the new login test is the only added occurrence in this PR's merge ref.

tests/unit/proxy/auth/test_login_utils.py

@@ -671,7 +671,7 @@ async def test_login_password_kdf_runs_off_event_loop_thread(monkeypatch):
             result = await authenticate_user(
                 username=user_email,
                 password=password,
-                master_key="sk-1234",
+                master_key="sk-master-for-tests",
                 prisma_client=mock_prisma_client,
                 throttle=_unlimited_throttle(),
                 general_settings=_POLICY_NO_BREACH_CHECK,