Enforce Optional[T] typing for None-defaulted Pydantic fields

[jeffreyksmithjr]

Reference Issues or PRs

See also: nebari-dev/nebari-docs#615 (companion docs PR)

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Documentation

• For new features or enhancements, a corresponding PR has been opened in the documentation repository (if applicable)
  • Link to docs PR: Add code conventions page to community docs nebari-docs#615

Testing

• Did you test the pull request locally?
• Did you add new tests?

How to test this PR?

Run the new enforcement test:

pytest tests/tests_unit/test_code_conventions.py -v

The test dynamically imports all modules under nebari and _nebari, collects
every Pydantic BaseModel subclass, and verifies that any field defaulting to
None includes NoneType in its resolved annotation. It handles Optional,
Union, T | None, and Annotated wrappers.

Any other comments?

Two existing violations were fixed:

• GCPClusterInputVars.node_group_image_type in stages/infrastructure/__init__.py
• Storage.type in stages/kubernetes_services/__init__.py

Both already had Optional imported in their respective files.