feat: add per-group expandable control to button dropdown

[revezard]

Description

Adds an expandable flag to ButtonDropdown's ItemGroup that overrides the dropdown-level
expandableGroups on a per-group basis:

• expandable: true — the group renders as an expandable submenu even when expandableGroups is unset
• expandable: false — the group renders inline (flat) even when expandableGroups is set
• unset — the group inherits the expandableGroups value

This lets a single dropdown mix expandable and flat groups (e.g. keep high-traffic groups inline
while long/secondary groups collapse into submenus). Expandability is resolved per node via a
small planeOf plane model rather than a single global flag, so expandable and flat groups
coexist correctly for keyboard navigation on both desktop and in the mobile restricted view.
The change is backward-compatible: with no expandable flag set, behavior is identical to today.

Implemented per design alignment with the Cloudscape team (<name / discussion or issue link>).

Related links, issue #, if available: <link, or n/a>

How has this been tested?

• Unit: full button-dropdown unit suite passes. Added is-group-expandable.test.ts (locks the
  inherit/override resolution) and new mixed expandable/flat navigation cases in
  move-highlight.test.ts covering both desktop plane confinement and the mobile restricted-view
  path (flat-group children reachable, collapsed expandable-group children gated).
• Accessibility (integration): the axe a11y suite passes on button-dropdown/expandable and a
  new button-dropdown/expandable-mixed dev page, in both light and dark modes.
• Manual: keyboard navigation on desktop and in forced-mobile mode; arrow-key traversal across
  flat groups and into/out of expandable groups; filtering flattens all groups; toggling
  expandableGroups with per-group expandable overrides behaves as documented.

Reviewers can exercise it on the button-dropdown/expandable-mixed dev page: toggle
expandableGroups and observe the forced-expandable (true) and forced-flat (false) groups hold
their behavior while the rest inherit.

Review checklist

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

Correctness

• Changes include appropriate documentation updates. (new expandable prop documented via JSDoc on ItemGroup)
• Changes are backward-compatible. (unset flag inherits expandableGroups; absent flag preserves current behavior)
• Changes do not include unsupported browser features.
• Changes were manually tested for accessibility. (keyboard nav desktop + mobile; axe suite light/dark)

Security

• N/A — this change does not handle URLs.

Testing

• Changes are covered with new/existing unit tests. (is-group-expandable.test.ts, move-highlight.test.ts mixed-mode cases)
• Changes are covered with new/existing integration tests. (axe a11y integ on the expandable + expandable-mixed dev pages)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.76%. Comparing base (6de850b) to head (c731f10).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4947      +/-   ##
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- Coverage   97.66%   93.76%   -3.91%     
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  Files         959      958       -1     
  Lines       31342    27144    -4198     
  Branches    11578     9763    -1815     
==========================================
- Hits        30610    25451    -5159     
+ Misses        686      613      -73     
- Partials       46     1080    +1034     

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