fix(client): drop the unused import CI's lint caught

Left behind when the async construction test was removed. ruff check now passes
on mem0/ and tests/.

Claude-Session: https://claude.ai/code/session_01C7tEmH86HAr7GoAAKCEHZb

tests/test_client_surface_headers.py

@@ -8,8 +8,6 @@ TypeError, and the whole suite stayed green because nothing built one.
 import os
 from unittest.mock import patch
 
-import pytest
-
 from mem0.client.main import _bounded_stack, _client_headers, _client_stack