feat: Add port availability check at startup

[andygoodluck]

Summary

Adds port availability check at the beginning of main.py to prevent port conflicts that only surface after lengthy custom node loading.

Problem

Users would start ComfyUI, wait for custom nodes to load (which can take a long time), and only then discover that the port was already in use:

# Custom nodes loading... (can take minutes)
...
# Finally, after all loading:
OSError: [Errno 48] Address already in use

This wastes time and provides a poor user experience.

Solution

Check port availability before loading custom nodes. If the port is in use, exit immediately with a clear error message.

Changes

1. main.py

• Added socket import
• Added port availability check before creating PromptServer
• Exits early with helpful error message if port is in use

2. port_check.py (new)

• Utility module with port checking functions
• check_port_available(): Check if a port is available
• check_server_port(): Check and exit if port is in use
• Can be reused for other port-related functionality

3. test_port_check.py (new)

• Comprehensive tests for port checking functionality
• Tests available ports, in-use ports, and timeout handling

Example Output

When port is available:

Port 8188 is available.
# ... continues loading custom nodes

When port is in use:

============================================================
PORT CONFLICT DETECTED
============================================================
Port 8188 on 0.0.0.0 is already in use.
Please either:
  1. Stop the other process using port 8188
  2. Start ComfyUI on a different port using: --port <PORT>
============================================================

Testing

python test_port_check.py

All tests pass:

• ✅ Available port detection
• ✅ In-use port detection
• ✅ Timeout handling

Related Issue

Fixes #8935