chore: Remove ResizeObserver polyfill

[TrevorBurnham]

All target browsers now support the native ResizeObserver API, making the polyfill unnecessary: https://developer.mozilla.org/en-US/docs/Web/API/ResizeObserver#browser_compatibility.

This change reduces bundle size and simplifies the codebase.

Changes:

• Remove @juggle/resize-observer from package.json dependencies
• Update sticky scrollbar hook to use native ResizeObserver
• Add global ResizeObserver mock in Jest setup for JSDOM compatibility
• Update mock file to use native ResizeObserver types with JSDOM-compatible implementations

Note that the mock file can be removed once the polyfill is also removed from @cloudscape-design/component-toolkit: cloudscape-design/component-toolkit#154

How has this been tested?

✅ All unit tests pass.

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 97.12%. Comparing base (f2d96fb) to head (ff64b1e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3790      +/-   ##
==========================================
- Coverage   97.12%   97.12%   -0.01%     
==========================================
  Files         844      844              
  Lines       24553    24553              
  Branches     8660     8659       -1     
==========================================
- Hits        23848    23846       -2     
- Misses        656      700      +44     
+ Partials       49        7      -42     

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[just-boris]

This change is good and passed all our internal tests. Ready to merge