* @return new referral, combining a chain of referrals
     */
    DfsReferralDataInternal combine(DfsReferralData next);

    /**
     * Append another referral to this referral chain
     *
     * @param dr the referral to append
     */
    void append(DfsReferralDataInternal dr);

        DfsReferralDataInternal mockReferral = mock(DfsReferralDataInternal.class);
        // Should not throw any exception.
        assertDoesNotThrow(() -> dfsImpl.cache(mockContext, "invalidpath", mockReferral));
    }

    @Test
    void testCache_ValidPath() {
        // Scenario: A valid referral is cached.
        DfsReferralDataInternal mockReferral = mock(DfsReferralDataInternal.class);