Add mock printer server for multi-printer testing

Simulates a Creality K2 Plus / Moonraker setup without real hardware:
- WebSocket server on port 9999 (boxsInfo, heartbeat protocol)
- Moonraker HTTP API on port 7125 (print_stats, gcode/script)
- Control API at /mock/* for triggering print jobs and slot changes

Run multiple instances on different loopback IPs (127.0.0.2, 127.0.0.3, ...)
to test the multi-printer setup locally.

Also includes mock_sshpass.sh: a fake sshpass wrapper that intercepts SSH
auto-link calls and fetches material_box_info from the mock HTTP endpoint
instead, enabling SSH auto-linking tests without a real SSH server.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>