Insert itemset declaration for range labels

[lognaturel]

Closes #835

Verified with Collect v2026.2.0 beta 1 using rangelabels.xlsx

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

In xls2json I ended up removing the tick_labelset parameter from the json representation and injecting an itemset using add_choices_info_to_question. That feels like a reasonable resulting json representation and a good way to reuse existing functionality.

For the conversion to xform, I considered whether I should reuse functionality between MultipleChoiceQuestion and RangeQuestion, maybe pulling out a mixin to represent having items. In the end I handled itemsets specifically for range because choices for selects are more complex. I think we may want to revisit this when we support reference geometry for geopoint/trace/shape which will also involve an itemset child.

I introduced a range_itemset testing function. It just checks the itemset declaration and not the label and value children, that didn't feel necessary.

What are the regression risks?

Limited to the range question type, probably really only around tick labels. Feels quite safe to me.

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

No.

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

[yanokwa]

LGTM