833: fix missing opening square bracket on unicode character set

[lindsay-stevens]

Closes #833

• test case includes characters in the broken range as an example (the test fails without the fix). The intent of the regex is to match XML so it should have allowed them.
• not quite a regression since although the typo has been present since 2024, the previous regex validation only allowed ascii characters. Also the error message just says "letters" are allowed rather than specifying that a particular type of letter is supported.

Why is this the best possible solution? Were any other approaches considered?

I think this is the only way to fix it? There is the option to only allow ASCII letters but that seems unnecessarily restrictive.

What are the regression risks?

Fixes a bug.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

N/A

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests
• run python -m unittest and verified all tests pass
• run ruff format pyxform tests and ruff check pyxform tests to lint code
• verified that any code or assets from external sources are properly credited in comments

[lognaturel]

Thank you!