Fix Namecheap non-MX preference normalization

Ignore Namecheap MXPref placeholders outside MX records, add round-trip regression coverage, and keep the declared pytest gate collectable.

tests/test_dns_manifest.py

@@ -5,7 +5,7 @@ from pathlib import Path
 import tempfile
 import unittest
 
-from ampg.dns import DNSZoneSnapshot, ProviderDNSRecord
+from ampg.dns import DNSZoneSnapshot, NamecheapDNSProvider, ProviderDNSRecord
 from ampg.dns_manifest import (
     load_dns_manifest,
     merge_dns_manifest_records,
@@ -60,6 +60,29 @@ class FakeProvider:
         self.records = tuple(records)
 
 
+class NamecheapProviderTests(unittest.TestCase):
+    def test_non_mx_preference_placeholder_is_ignored(self):
+        response = """\
+<ApiResponse Status="OK">
+  <CommandResponse>
+    <DomainDNSGetHostsResult>
+      <host Name="shop" Type="A" Address="192.0.2.1" TTL="300" MXPref="10" />
+      <host Name="@" Type="MX" Address="mx.example" TTL="300" MXPref="20" />
+    </DomainDNSGetHostsResult>
+  </CommandResponse>
+</ApiResponse>
+"""
+        provider = NamecheapDNSProvider.__new__(NamecheapDNSProvider)
+        provider._request = lambda _command, _params: response
+        provider._domain_params = lambda _domain: {"SLD": "example", "TLD": "test"}
+
+        snapshot = provider.get_hosts("example.test")
+
+        self.assertEqual("ok", snapshot.status)
+        self.assertIsNone(snapshot.records[0].mx_pref)
+        self.assertEqual(20, snapshot.records[1].mx_pref)
+
+
 class DNSManifestTests(unittest.TestCase):
     def setUp(self):
         self.temporary = tempfile.TemporaryDirectory()

tests/test_service_manifest.py

@@ -22,7 +22,7 @@ def golden() -> dict:
     return json.loads(GOLDEN_PATH.read_text(encoding="utf-8"))
 
 
-def test_sign(secret: int, message: bytes) -> tuple[str, str]:
+def _sign_for_test(secret: int, message: bytes) -> tuple[str, str]:
     point = bip340.point_mul(secret, bip340.SECP256K1_GENERATOR)
     assert point is not None
     adjusted = secret if point[1] % 2 == 0 else bip340.SECP256K1_ORDER - secret
@@ -138,7 +138,7 @@ class ServiceManifestTest(unittest.TestCase):
         manifest = deepcopy(golden()["manifest"])
         root_secret = int(golden()["test_secret_key"], 16)
         signing_secret = 4
-        signing_key, _ = test_sign(signing_secret, bytes(32))
+        signing_key, _ = _sign_for_test(signing_secret, bytes(32))
         delegation_payload = {
             "schema": sm.DELEGATION_SCHEMA,
             "service_id": manifest["payload"]["service_id"],
@@ -148,10 +148,10 @@ class ServiceManifestTest(unittest.TestCase):
             "expires_at": "2037-01-01T00:00:00Z",
             "sequence": 1,
         }
-        root_key, delegation_signature = test_sign(
+        root_key, delegation_signature = _sign_for_test(
             root_secret, sm.delegation_digest(delegation_payload)
         )
-        signing_key, manifest_signature = test_sign(
+        signing_key, manifest_signature = _sign_for_test(
             signing_secret, sm.service_manifest_digest(manifest["payload"])
         )
         manifest["delegation"] = {