fix: vertical centering for h2 headers without description in expandable sections

[Who-is-PS]

Description

• Added CSS rule to vertically center h2 headers without descriptions in expandable section
• Ensures consistent vertical alignment between header text and action buttons/icons across all expandable section

Related links, issue #, if available:
AWSUI-61245

How has this been tested?

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 97.17%. Comparing base (e5e2ffc) to head (18ed291).
⚠️ Report is 35 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3944      +/-   ##
==========================================
+ Coverage   97.15%   97.17%   +0.01%     
==========================================
  Files         854      856       +2     
  Lines       24960    25242     +282     
  Branches     8792     8940     +148     
==========================================
+ Hits        24251    24528     +277     
- Misses        660      665       +5     
  Partials       49       49              

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