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_mcp_server.py

@@ -1,12 +1,13 @@
 """Tests for the MCP server module."""
 
+import hashlib
 import json
 import os
-import hashlib
 from unittest.mock import MagicMock, mock_open, patch
 
-import yaml
 import pytest
+import yaml
+
 from penpot_mcp.server.mcp_server import PenpotMCPServer, create_server