fix(a11y): make keyboard focus visible on every AG-Grid table (#14627)

fix(a11y): show a keyboard focus ring on every AG-Grid cell (LE-2233)

The generic grid theme replaced the focus outline on .ag-cell-focus with a
1px #94a3b8 border and a 5%-alpha shadow, and .ag-tool-mode / .no-border /
.cell-disable-edit cells and AG text inputs cleared it entirely, so keyboard
focus in Settings tables, the table modal and the tools modal had no
perceivable indicator (WCAG 2.4.7). Only .ag-no-border and .ag-knowledge-table
grids had a :focus-visible ring (LE-1561).

- scope every outline suppression to :focus:not(:focus-visible) so the mouse
  look is unchanged and keyboard focus falls through
- replace the two per-grid rings with one .ag-root-wrapper :focus-visible rule
  for cells, header cells, rows and AG inputs, using the --ring token
- hide AG-Grid's own inner ::after header box now that the ring replaces it
- drop the ring suppression from the unused button "menu" variant
- delete two suppression blocks that could never apply (text inputs are always
  :focus-visible; `input .ag-cell-edit-input` matched nothing)
- focus-visible.spec.ts: one perceivability helper (opaque outline, box-shadow
  with a >=0.5-alpha layer, or AG's ::after border) shared by all tests, plus
  an API-keys grid test that fails on release-1.12.0 and passes here