feat: add defaultExpanded and onToggle to stacked Flashbar

[ernst-dev]

Description

Adds two opt-in, backward-compatible props to Flashbar for controlling the stacked notifications group (stackItems), filling a gap in the existing stacking/collapse behavior — previously the expand/collapse state was purely internal and could neither be initialized nor observed by consumers.

• defaultExpanded?: boolean — sets the initial expanded/collapsed state of the stacked group. Only has effect when stackItems is true. Defaults to false, so existing behavior is unchanged.
• onToggle?: NonCancelableEventHandler<FlashbarProps.ToggleDetail> — fires with { expanded } when the user expands or collapses the stack via the notification bar.

Implementation reuses the existing collapsible-flashbar expand/collapse mechanics (initializes isFlashbarStackExpanded from defaultExpanded, fires onToggle from the existing toggleCollapseExpand handler). No changes to the non-stacked Flashbar.

Ticket note: This work references SIM AWSUI-61751. That ticket is actually a resolved "Cloudscape a11y feedback request" and does not describe the Flashbar stack/collapse feature. The Flashbar already ships stackItems (collapsible stacking), so — per the task guidance to fill the described gap rather than duplicate — this PR adds programmatic control/observation of the existing collapse state instead of re-implementing stacking.

Related links, issue #, if available: SIM AWSUI-61751 (mismatched — see note above)

How has this been tested?

• New unit tests in src/flashbar/__tests__/collapsible.test.tsx covering: default-collapsed, defaultExpanded true/false, onToggle firing with expanded: true/false, and no callback on initial render.
• Full Flashbar suite + documenter snapshot: 324 tests / 97 snapshots passing.
• eslint clean on all changed files; gulp quick-build succeeds.
• New dev page pages/flashbar/stack-collapse-control.page.tsx for manual/visual verification.

Review checklist

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

Correctness

• Changes include appropriate documentation updates. (prop TSDoc + API definition snapshot)
• Changes are backward-compatible. (both props optional; defaults preserve current behavior)
• Changes do not include unsupported browser features.
• Changes were manually tested for accessibility. (aria-expanded on the toggle reflects the new initial state)

Security

• No URL handling in this change.

Testing

• Changes are covered with new/existing unit tests? Yes.
• Changes are covered with new/existing integration tests? Existing collapsible integ coverage still applies.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.61%. Comparing base (f97729a) to head (5e469ea).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4811   +/-   ##
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  Coverage   97.61%   97.61%           
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  Files         952      952           
  Lines       30816    30818    +2     
  Branches    11318    11318           
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+ Hits        30081    30083    +2     
- Misses        688      728   +40     
+ Partials       47        7   -40     

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