fix: Resolve lint and test failures in CI

- Remove unused `_singleton_connection` import in __init__.py (F401)
- Remove unused `os` import in test_thread_safe.py (F401)
- Remove unused `Callable` import in connection.py (F401)
- Fix mock_cache fixture: `cache` → `download_path` for 2.0 settings

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/__init__.py

@@ -130,7 +130,7 @@ def conn(
     ThreadSafetyError
         If thread_safe mode is enabled.
     """
-    from .instance import _singleton_connection, _check_thread_safe, _global_config
+    from .instance import _check_thread_safe, _global_config
     import datajoint.instance as instance_module
 
     _check_thread_safe()

tests/unit/test_thread_safe.py

@@ -1,7 +1,5 @@
 """Tests for thread-safe mode functionality."""
 
-import os
-
 import pytest