fix: DH-21258: Fix column group separators

[vbabich]

Update GridUtils.getColumnSeparatorIndex to take column group depth into account.

BREAKING CHANGE: getColumnSeparatorIndex now requires a model argument.

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.24%. Comparing base (fa61b3b) to head (a4bd04e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/grid/src/GridUtils.ts	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2597      +/-   ##
==========================================
- Coverage   45.48%   45.24%   -0.25%     
==========================================
  Files         771      771              
  Lines       43591    43604      +13     
  Branches    11027    11035       +8     
==========================================
- Hits        19827    19727     -100     
- Misses      23748    23861     +113     
  Partials       16       16              

Flag	Coverage Δ
unit	45.24% <86.66%> (-0.25%)	⬇️

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[bmingles]

LGTM