feat: wrap expandable section header actions on narrow widths

[ernst-dev]

Description

Allow ExpandableSection header actions (the headerActions slot) to wrap gracefully on narrow container widths instead of overflowing/clipping.

For the non-container variants that render the headerActions slot (default, inline), the header row (.header-actions-wrapper) already sets flex-wrap: wrap, but the heading (.header-wrapper) could not shrink below its content min-size (min-width: auto). On narrow widths this pushed the actions off-screen / clipped the content rather than degrading gracefully.

This change lets the heading shrink and wrap its text (flex: 1 1 auto + min-inline-size: 0 down the title flex chain, plus overflow-wrap: break-word on the header text). The result:

• On moderately narrow widths the header text wraps first while the actions stay inline.
• On very narrow widths the actions wrap underneath the title — instead of overflowing or clipping.

Scope is limited to the .header-actions-wrapper path (default / inline variants). The container variant (which routes through the Header component), navigation, and the compact variant (chart popovers, intentionally non-wrapping) are untouched. No public API / prop was added — this is a pure, backward-compatible layout hardening of an existing responsive behavior.

Related links, issue #, if available: AWSUI-60837

Ticket-vs-description note: The SIM ticket AWSUI-60837 ("Allow expandable section header actions to stay inline, wrap header text instead") frames the ask as keeping actions inline and wrapping the header text, and mentions the container-header responsive breakpoint being "too small when widths are always going to be small". The task description for this change frames it as "actions wrap under the title on narrow widths instead of overflowing". Both intents converge on this SCSS fix: the heading now wraps its text first (actions stay inline), and only when there is genuinely no room do the actions drop below the title. The container-variant breakpoint tuning (owned by the Header component) is intentionally out of scope here and noted for follow-up.

How has this been tested?

• npm run quick-build — green
• Unit tests: jest src/expandable-section/__tests__ — 182 passed (4 new tests covering the header-actions wrapper for the default and inline variants, and absence of the wrapper when there are no actions)
• eslint and stylelint on the changed files — green
• Added a dev page (pages/expandable-section/header-actions-wrap.page.tsx) rendering the sections at 640 / 360 / 240 / 160 px to visually verify wrapping behavior

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 97.61%. Comparing base (f97729a) to head (cc1282f).

Additional details and impacted files

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##             main    #4804   +/-   ##
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  Coverage   97.61%   97.61%           
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  Files         952      952           
  Lines       30816    30816           
  Branches    11318    11318           
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  Hits        30081    30081           
  Misses        688      688           
  Partials       47       47           

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