Reject blank required probing axes

[righteousgambit]

Closes #671

Summary

• treat empty and whitespace-only X/Y values as missing when the selected operation conditionally requires that axis
• update Bore, Boss, Angle, and the three ProbeTip operation variants consistently
• retain valid numeric strings, including "0"
• add a matrix regression test and an unreleased changelog entry

This does not change which operations require which axes; it only makes the existing requirement check agree with the G-code formatter's treatment of blank values.

Validation

Validated against develop@2ea66c10e96d88013a04441134d3d4f8e8c88510 with Python 3.11:

• regression suite on the base: 24 failed, 24 passed (every blank/whitespace case failed; every numeric-string control passed)
• regression suite with this change: 48 passed
• ruff check passed for all changed Python files
• git diff --check passed

Targeted command:

poetry run python -m pytest -q tests/unit/test_probing_required_axes.py

Changelog

- Fixed: Treat blank conditionally required X/Y probing inputs as missing

Real branch verification

The repository's locked Poetry environment was installed from poetry.lock. The targeted suite passed on real fork commit 74f380c9de46, whose sole parent is pinned upstream develop@2ea66c10e96d88013a04441134d3d4f8e8c88510.

poetry run python -m pytest -q tests/unit/test_probing_required_axes.py

Result: 48 passed.

Quality gates on the real branch:

• Ruff check: passed
• Ruff format --check: passed
• git diff --check 2ea66c10e96d88013a04441134d3d4f8e8c88510..74f380c9de466cc317653de86a361ffc0261f236: passed