Adds port availability check before loading custom nodes to prevent
port conflicts that only surface after lengthy custom node loading.
Changes:
- Added socket import for port checking
- Added port availability check in main.py before creating PromptServer
- Created port_check.py utility module with helper functions
- Created test_port_check.py with comprehensive tests
The check will:
1. Verify the server port is available before any heavy loading
2. Exit early with clear error message if port is in use
3. Provide helpful instructions for resolving the conflict
Fixes#8935
[Bounty]
