chore: Add new inline-code variant in Box

[at-susie]

Description

We identified the lack of a consistent visual style for inline code elements across services. In the absence of a standardized approach, teams have implemented custom styles, resulting in visual inconsistencies.

This PR introduces a new variant in Box component called awsui-inline-code, which can be used to display a unified style for inline-code elements.

Related links, issue #, if available: n/a

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 (a943c56) to head (bc21969).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3849   +/-   ##
=======================================
  Coverage   97.17%   97.17%           
=======================================
  Files         850      850           
  Lines       24772    24774    +2     
  Branches     8729     8730    +1     
=======================================
+ Hits        24071    24073    +2     
  Misses        694      694           
  Partials        7        7           

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