style: fix ruff formatting issues in conftest.py

- Fix import sorting and organization
- Remove trailing whitespace
- Add proper newline at end of file

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

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

tests/conftest.py

@@ -1,6 +1,7 @@
 """Pytest configuration and fixtures for LEANN tests."""
 
 import os
+
 import pytest
 
 
@@ -19,9 +20,10 @@ def cleanup_session():
 
     # Basic cleanup after all tests
     try:
-        import psutil
         import os
 
+        import psutil
+
         current_process = psutil.Process(os.getpid())
         children = current_process.children(recursive=True)