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.

penpot_mcp/utils/cache.py

@@ -3,7 +3,8 @@ Cache utilities for Penpot MCP server.
 """
 
 import time
-from typing import Optional, Dict, Any
+from typing import Any, Dict, Optional
+
 
 class MemoryCache:
     """In-memory cache implementation with TTL support."""