fix: select, multiselect option grouping

[SpyZzey]

Description

This PR fixes a bug that groups "root"-level options in a select and multiselect dropdown to the previous group. The issue was non-visual, but leads to issues with accessibility screen readers.

• Added Tests
• Removed a condition in "unflatten-options" that wrongly grouped "non"-child elements to the previous group.

Related Issues: [AWSUI-61453]

How has this been tested?

• Added unit tests to test the unflattenOptions function.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.99%. Comparing base (abb494e) to head (ce9c3db).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4038   +/-   ##
=======================================
  Coverage   96.99%   96.99%           
=======================================
  Files         861      861           
  Lines       25207    25207           
  Branches     9098     9097    -1     
=======================================
  Hits        24450    24450           
- Misses        710      751   +41     
+ Partials       47        6   -41     

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[pan-kot]

What is the case for the option.type to be undefined?

[jperals]

Assuming that root-level non-parent nodes have option.type set to undefined, I think this can be on the right direction, and simpler than my approach.

Could you add some testing coverage?

[SpyZzey]

What is the case for the option.type to be undefined?

@pan-kot
In the flatten-options.ts, the option.type = "child" will be set, if the option is inside the options array of another parent-option (e.g. groups). The parent-option will get the option.type = "parent" set.

@jperals Exactly. I will add some test cases.