test: add resolver-level regression tests for DNSCACHE_ENABLED=False

Add two regression tests that verify DNS results are not stored in
dnscache when DNSCACHE_ENABLED=False (cache_size=0):

- test_caching_hostname_resolver_dnscache_disabled_rejects_storage:
  verifies _CachingResolutionReceiver does not write to dnscache
  when CachingHostnameResolver is initialized with cache_size=0

- test_caching_threaded_resolver_dnscache_disabled_rejects_storage:
  verifies dnscache rejects storage at the LocalCache level
  when limit=0

tests/test_resolver.py

@@ -53,3 +53,34 @@ def test_caching_hostname_resolver_no_addresses_not_cached():
     resolver.resolveHostName(Mock(), "example.com")
 
     assert "example.com" not in dnscache
+
+
+def test_caching_hostname_resolver_dnscache_disabled_rejects_storage():
+    # Regression test for LocalCache(limit=0) rejecting storage
+    # when DNSCACHE_ENABLED=False. _CachingResolutionReceiver writes
+    # directly to dnscache, so we verify nothing is stored.
+    def fake_resolve(receiver, *_):
+        receiver.resolutionBegan(Mock())
+        receiver.addressResolved(Mock())
+        receiver.resolutionComplete()
+        return receiver
+
+    reactor = Mock()
+    reactor.nameResolver.resolveHostName.side_effect = fake_resolve
+
+    # cache_size=0 simulates DNSCACHE_ENABLED=False
+    resolver = CachingHostnameResolver(reactor, cache_size=0)
+    resolver.resolveHostName(Mock(), "example.com")
+
+    # dnscache should remain empty despite address being resolved
+    assert "example.com" not in dnscache
+    assert len(dnscache) == 0
+
+
+def test_caching_threaded_resolver_dnscache_disabled_rejects_storage():
+    # Regression test: when DNSCACHE_ENABLED=False, resolved DNS results
+    # should not be stored in dnscache even after successful resolution.
+    dnscache.limit = 0
+    dnscache["example.com"] = "1.2.3.4"
+    assert "example.com" not in dnscache
+    assert len(dnscache) == 0