chore: Stable hashes for border tokens

[pan-kot]

Description

A follow-up for cloudscape-design/theming-core#190

How has this been tested?

• Existing screenshot tests

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 97.59%. Comparing base (726348b) to head (47e8275).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4718   +/-   ##
=======================================
  Coverage   97.59%   97.59%           
=======================================
  Files         950      950           
  Lines       30662    30662           
  Branches    11240    11240           
=======================================
  Hits        29926    29926           
  Misses        689      689           
  Partials       47       47           

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