Add test_credentials.py for Penpot API credential verification and project listing

- Introduced a new script, `test_credentials.py`, to verify Penpot API credentials and list associated projects.
- The script loads environment variables, checks for required credentials, and attempts to authenticate with the Penpot API.
- Added functionality to fetch and display project details and files, including error handling for authentication and project retrieval.
- Updated `PenpotAPI` class to include a User-Agent header and improved error handling during profile retrieval.
- Minor adjustments in import order across various modules for consistency.

tests/test_cache.py

@@ -3,9 +3,12 @@ Tests for the memory caching functionality.
 """
 
 import time
+
 import pytest
+
 from penpot_mcp.utils.cache import MemoryCache
 
+
 @pytest.fixture
 def memory_cache():
     """Create a MemoryCache instance with a short TTL for testing."""