fix(subscriptions): use domain-specific error constant in updateCategory test

Matches the parallel updateTopic error test on the same suite.

Addresses review comment on PR #517.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sarthak688

tests/unit/services/notification/subscriptions.test.ts

@@ -208,7 +208,7 @@ describe('SubscriptionService Unit Tests', () => {
     });
 
     it('should propagate errors', async () => {
-      mockApiClient.post.mockRejectedValue(createMockError(TEST_CONSTANTS.ERROR_MESSAGE));
+      mockApiClient.post.mockRejectedValue(createMockError(NOTIFICATION_TEST_CONSTANTS.ERROR_SUBSCRIPTION_INVALID));
 
       await expect(
         subscriptionService.updateCategory(NOTIFICATION_TEST_CONSTANTS.TENANT_ID, [
@@ -219,7 +219,7 @@ describe('SubscriptionService Unit Tests', () => {
             notificationMode: NotificationMode.InApp,
           },
         ])
-      ).rejects.toThrow(TEST_CONSTANTS.ERROR_MESSAGE);
+      ).rejects.toThrow(NOTIFICATION_TEST_CONSTANTS.ERROR_SUBSCRIPTION_INVALID);
     });
   });
 });