Fix cross-platform compatibility, calibration tolerances, and gantry motion boundaries for Flex gRPC testing

[SR-Green]

This Pull Request introduces fixes for Flex gRPC integration and HITL tests on real hardware and Windows development hosts:

1. Adjusted pytest.approx tolerances to account for real deck calibration offsets.
2. Inverted Z jog direction in HITL motion tests to jog down (-5mm) first (since Z=250mm is the physical home limit on real hardware).
3. Fixed cross-platform path slashes in modules.py.
4. Added client test control script with safety checks.