feat: DH-21829: add invisible accessibility layer to Grid for e2e testing

[mofojed]

• Add GridAccessibilityLayer component that renders DOM elements overlaid on canvas
• Data cells use data-testid="grid-cell-{column}-{row}"
• Column headers use data-testid="grid-column-header-{column}-{depth}"
• Row headers use data-testid="grid-row-header-{row}" (when rowHeaderWidth > 0)
• Add enableAccessibilityLayer prop to Grid and IrisGrid
• All elements have pointer-events: none to allow clicks through to canvas
• Add e2e tests for accessibility layer functionality

[dsmmcken]

My 3 concerns:

1. https://www.w3.org/WAI/ARIA/apg/patterns/grid/ should come as close as possible to adhering to this ARIA spec, I see you already correctly have role=grid.
2. If it's on by default it should thoroughly tested to check for performance regressions. Check against main fully screen grids too, multiple small grids. Fast updating raw grids.
3. Should it be nested INSIDE the canvas instead of adjacent. I don't know if there's a benefit one way or the other, just asking.

[codecov]

Codecov Report

❌ Patch coverage is 93.54839% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.58%. Comparing base (efd24a7) to head (232fb64).

Files with missing lines	Patch %	Lines
packages/grid/src/GridAccessibilityLayer.tsx	94.64%	3 Missing ⚠️
packages/grid/src/Grid.tsx	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2630      +/-   ##
==========================================
- Coverage   49.77%   49.58%   -0.20%     
==========================================
  Files         774      775       +1     
  Lines       43770    43832      +62     
  Branches    11258    11091     -167     
==========================================
- Hits        21788    21735      -53     
- Misses      21936    22079     +143     
+ Partials       46       18      -28     

Flag	Coverage Δ
unit	49.58% <93.54%> (-0.20%)	⬇️

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[dgodinez-dh]

Checked out the branch:

• ran e2e tests which passed
• ran the ui
• confirmed I see the accessibility layer in inspect
• confirmed it filters as expected