fix: avoid MySQL timezone dependency in billing admin

apps/billing/tests.py

@@ -10,6 +10,7 @@ from django.contrib.auth import get_user_model
 from django.core.exceptions import ValidationError
 from django.db import IntegrityError, OperationalError, connections, transaction
 from django.test import TestCase, TransactionTestCase, override_settings
+from django.urls import reverse
 from django.utils import timezone
 
 from apps.ai.models import AiModel, ModelAlias
@@ -49,6 +50,30 @@ from apps.users.models import ApiKey, UserWallet
 TEST_ENCRYPTION_KEY = Fernet.generate_key().decode("ascii")
 
 
+class BillingAdminTests(TestCase):
+    def setUp(self):
+        user_model = get_user_model()
+        self.admin_user = user_model.objects.create_superuser(
+            username="billing-admin",
+            email="billing-admin@example.com",
+            password="test-password",
+        )
+        self.client.force_login(self.admin_user)
+
+    def test_exchange_rate_changelist_renders_when_rates_exist(self):
+        ExchangeRate.objects.create(
+            currency="CNY",
+            points_per_unit=Decimal("10.0000"),
+            effective_from=timezone.now(),
+            is_active=True,
+        )
+
+        response = self.client.get(reverse("admin:billing_exchangerate_changelist"))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "CNY")
+
+
 class BillingCoreModelTests(TestCase):
     def setUp(self):
         suffix = uuid.uuid4().hex[:8]