fix(regression): resolve getHeaderColumn with hidden leading columns

[ghiscoding]

fixes #2584

vibe coded a fix with copilot using GPT-5.3-Codex

TODOs

• modify 1 example to pre-init a grid with a column that has hidden: true in at least 1 example in the column definitions and modify Cypress test accordingly
• found another bug when hidding the first column and grouping by the second column, the group doesn't show even though it should
• also need to verify Export Excel/PDF

Screencast_20260513_012129.webm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (e6f9826) to head (eea71df).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2585   +/-   ##
=======================================
  Coverage   100.0%   100.0%           
=======================================
  Files         197      197           
  Lines       25348    25355    +7     
  Branches     8957     8961    +4     
=======================================
+ Hits        25348    25355    +7     

Flag	Coverage Δ
angular	100.0% <ø> (ø)
universal	100.0% <100.0%> (ø)

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commit: eea71df