fix: Cherry-pick DH-21258: Fix column group separators

[vbabich]

Cherry-pick #2597

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

BREAKING CHANGE: getColumnSeparatorIndex now requires a model argument.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.44%. Comparing base (bb592e9) to head (fbf1ac7).
⚠️ Report is 1 commits behind head on release/v0.85.

Files with missing lines	Patch %	Lines
packages/grid/src/GridUtils.ts	60.00%	6 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           release/v0.85    #2599   +/-   ##
==============================================
  Coverage          48.44%   48.44%           
==============================================
  Files                712      712           
  Lines              39747    39760   +13     
  Branches            9921     9929    +8     
==============================================
+ Hits               19255    19263    +8     
- Misses             20479    20484    +5     
  Partials              13       13           

Flag	Coverage Δ
unit	48.44% <60.00%> (+<0.01%)	⬆️

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[bmingles]

LGTM