chore: Use getOptionalElementWidth in top navigation for better type safety

[jperals]

Description

Ticket: AWSUI-62169

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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[Copilot]

Pull request overview

This PR updates Top Navigation sizing measurements to use the existing getOptionalElementWidth helper when reading widths of the search utility and overflow menu trigger, improving type safety and avoiding unsafe non-null assertions in DOM querying.

Changes:

• Replaced direct querySelector(... )!.getBoundingClientRect().width calls with getOptionalElementWidth(...) for optional utility elements.
• Ensures missing elements are handled consistently by returning 0 rather than throwing at runtime.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.62%. Comparing base (ee9b849) to head (20260a9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4836   +/-   ##
=======================================
  Coverage   97.62%   97.62%           
=======================================
  Files         956      956           
  Lines       31012    31012           
  Branches    11410    11410           
=======================================
  Hits        30277    30277           
  Misses        688      688           
  Partials       47       47           

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