feat: Table Column Grouping - Empty Cell Approach

[NathanZlion]

Description

[POC - WIP]
Related #4275

Related:

Related links, issue #, if available: n/a

How has this been tested?

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

❌ Patch coverage is 64.46402% with 242 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.61%. Comparing base (d21a76f) to head (6d75032).
⚠️ Report is 39 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/collection-preferences/content-display/utils.ts	1.94%	92 Missing and 9 partials ⚠️
src/table/use-column-widths.tsx	36.14%	46 Missing and 7 partials ⚠️
src/table/header-cell/group-header-cell.tsx	59.00%	37 Missing and 4 partials ⚠️
src/table/thead.tsx	80.95%	20 Missing ⚠️
...c/collection-preferences/content-display/index.tsx	34.78%	15 Missing ⚠️
src/table/column-grouping-utils.ts	96.96%	5 Missing ⚠️
src/table/table-role/grid-navigation.tsx	84.61%	3 Missing and 1 partial ⚠️
src/table/table-role/utils.ts	93.33%	2 Missing ⚠️
src/table/header-cell/hidden-header-cell.tsx	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4303      +/-   ##
==========================================
- Coverage   97.42%   96.61%   -0.82%     
==========================================
  Files         893      901       +8     
  Lines       26121    27027     +906     
  Branches     9437     9740     +303     
==========================================
+ Hits        25449    26112     +663     
- Misses        666      851     +185     
- Partials        6       64      +58     

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