fix: Enlarge SideNavigation expandable caret pointer target to 24px (WCAG 2.5.8)

[Harsh-Anand-Singh]

Description

Fixes a WCAG 2.5.8 (Target Size Minimum) finding: the SideNavigation expandable-link-group chevron caret button had a pointer target only as wide as its icon glyph, 12px in One Theme, 16px in Classic/Visual Refresh, below the 24px minimum.

This is a width-only change. The vertical target was already conformant: the caret button stretches to the full row height (28px comfortable / 24px compact) via flex stretch.

The clickable box is widened to 24px using inline padding compensated by an equal negative margin, so the extra width lands entirely in the adjacent non-interactive gutter:

• Start icon position (default): grows into the ~20px leading gutter (the space-l negative-margin seating).
• End icon position (withIcons): grows into the trailing gutter instead, because the label link's own click box (its negative-margin overhang) sits flush against the caret's inline-start edge, growing start-ward there would capture the link's clicks.

Zero layout shift: the caret glyph and the label link do not move (verified pixel-identical positions before/after in both themes), and pixel probes at the caret/link boundary confirm click ownership is unchanged. Behavior is the same across Classic, Visual Refresh, and One Theme, the tokens involved resolve identically; only the padding delta differs because the icon glyph is 16px vs 12px. Stacked link groups tile cleanly (24px target ≤ 24px compact row pitch), with no target overlap between rows.

Related links, issue #, if available: n/a

How has this been tested?

• Measured in devtools on the SideNavigation permutations dev page across One Theme / Visual Refresh, comfortable / compact, and both expand icon positions: caret button box is 24×28 (comfortable) and 24×24 (compact) after the change; glyph and label positions unchanged; before/after pixel probes at the caret/label boundary show identical click ownership.
• Functional check: clicking the newly added far-edge pixels toggles the section without triggering the header link.
• Existing unit suites: src/expandable-section + src/side-navigation (365 tests) pass.
• Existing integration suites: src/expandable-section/__integ__ + src/side-navigation/__integ__ pass.
• No new unit test: pure target-size CSS change, matching the inline-icon-pointer-target precedent; coverage is the a11y audit re-run plus existing permutation screenshots (visual regression runs in CI on this PR).

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.66%. Comparing base (f066dd8) to head (1dba7ac).

Additional details and impacted files

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##             main    #4907   +/-   ##
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  Coverage   97.66%   97.66%           
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  Files         958      958           
  Lines       31317    31317           
  Branches    11566    11566           
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  Hits        30585    30585           
  Misses        686      686           
  Partials       46       46           

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