style: import Callable from collections.abc (ruff UP035)

An auto-fix left uncommitted by the test-quality sweep; CI lint-ci (no
autofix) caught the committed typing.Callable import.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lesnik512

tests/test_error_mapping_terminal.py

@@ -1,7 +1,7 @@
 """Tests for the AsyncClient and Client internal terminal's exception mapping."""
 
+from collections.abc import Callable
 from http import HTTPStatus
-from typing import Callable
 
 import httpx2
 import pytest