test: add admin customer service tests and update conftest

- Add unit tests for admin customer service
- Update test configuration in conftest.py

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/conftest.py

@@ -128,5 +128,6 @@ pytest_plugins = [
     "tests.fixtures.vendor_fixtures",
     "tests.fixtures.customer_fixtures",
     "tests.fixtures.marketplace_import_job_fixtures",
+    "tests.fixtures.message_fixtures",
     "tests.fixtures.testing_fixtures",
 ]