fix: restore legacy dropdown sizing behavior

[mxschll]

Description

This PR restores legacy behavior in Select, Multiselect and Autosuggest when expandToViewport=true.

Current behavior: When expandToViewport=true, the dropdown’s width scales up to the xss breakpoint but can be smaller than the trigger if the content inside is narrower.

Expected behavior: When expandToViewport=true, the dropdown’s width matches the width of the trigger element, but only up to the xss breakpoint. This behavior is consistent with the legacy implementation (see AWSUI-19898).

We were missing a specific test case where the dropdown content is narrower than the trigger, which caused this behavior change to slip through in #4239.

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

How has this been tested?

Pipeline is green (will add tests for that specific scenario).

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 97.42%. Comparing base (5cfcc6d) to head (cee4dbe).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4283   +/-   ##
=======================================
  Coverage   97.42%   97.42%           
=======================================
  Files         891      892    +1     
  Lines       26098    26116   +18     
  Branches     9435     9437    +2     
=======================================
+ Hits        25426    25444   +18     
- Misses        629      666   +37     
+ Partials       43        6   -37     

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