Handle optional NXDL dimensions during validation

[CAOShurong]

Summary

• preserve each NXDL dimension's explicit required attribute when converting XML definitions
• skip a missing trailing dimension when the definition marks it optional (false or 0)
• keep the existing validation behavior for dimensions whose required attribute is omitted or true

Closes #248.

Verification

• pytest tests/test_validate.py -vv (6 passed)
• pytest --basetemp <E: scratch directory> (113 passed)
• ruff check src/nexusformat/nexus/utils.py src/nexusformat/nexus/validate.py tests/test_validate.py
• changed-file pre-commit hooks
• sdist and wheel build
• clean-wheel probe confirming an NXDL optional fourth dimension is accepted for a three-dimensional field
• installed nxcheck --help entry point

Disclosure: This change was developed with assistance from OpenAI Codex and reviewed and verified by CAOShurong.

[rayosborn]

Thank you for the PR, which is very timely, since I want to raise the variable dimensions issues at the NIAC meeting next week. It looks fine, but I need to run a couple of tests before approving.

[rayosborn]

I have made a number of other changes to the validation tools, including the addition of two new scripts, nxvalidate, which is equivalent to nxcheck -a, and nxlint to check for XML errors in NXDl files. This PR doesn't conflict with those changes and looks to fix an important shortcoming, so thanks for submitting it.

You might also be interested in changes I have proposed to the NeXus definitions (nexusformat/definitions#1664), which I believe clean up some of the issues surrounding dimension symbols in NXDL files. This has been discussed at the NIAC, but not approved yet.