test: add tests for nat64.py (#26)

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

tests/test_nat64.py

@@ -0,0 +1,51 @@
+from __future__ import annotations
+
+import unittest.mock
+
+import dns.exception
+import pytest
+
+from netsleuth.probes.nat64 import detect_nat64
+
+
+@pytest.fixture
+def mock_resolver(monkeypatch):
+    mock_resolve = unittest.mock.AsyncMock()
+    # We patch the Resolver class's resolve method
+    monkeypatch.setattr("dns.asyncresolver.Resolver.resolve", mock_resolve)
+    return mock_resolve
+
+
+@pytest.mark.asyncio
+async def test_detect_nat64_returns_prefix_on_success(mock_resolver):
+    # Mock Answer objects with string representation that match nat64_prefix_from_aaaa logic.
+    # The AAAA should encode one of the _IPV4ONLY_ARPA_A (192.0.0.170, 192.0.0.171).
+    # 64:ff9b::c000:aa is the well-known prefix for 192.0.0.170
+    class MockAnswer:
+        def __str__(self):
+            return "64:ff9b::c000:aa"
+
+    mock_resolver.return_value = [MockAnswer()]
+
+    result = await detect_nat64(timeout=1.0)
+    assert result == "64:ff9b::/96"
+
+
+@pytest.mark.asyncio
+async def test_detect_nat64_returns_none_if_no_synthesized_answer(mock_resolver):
+    class MockAnswer:
+        def __str__(self):
+            return "2606:4700:4700::1111"
+
+    mock_resolver.return_value = [MockAnswer()]
+
+    result = await detect_nat64(timeout=1.0)
+    assert result is None
+
+
+@pytest.mark.asyncio
+async def test_detect_nat64_returns_none_on_dns_exception(mock_resolver):
+    mock_resolver.side_effect = dns.exception.Timeout
+
+    result = await detect_nat64(timeout=1.0)
+    assert result is None