fix: Addresses styles-check flakiness

[pan-kot]

Description

The styles check util validates the presence of the component styles on the page and logs an error like Missing AWS-UI CSS for theme... to the console when the styles are not present.

This util causes integ tests flakiness with over 10 recorded occurrences.

The PR addresses the problem by making changes to the util so that it validates that the stylesheets (present upon document readiness) are loaded.

How has this been tested?

• New unit tests to cover the util changes
• Dry-run to live
• 400+ test runs with React v16 and v18 of the most affected tests: there was not a single occurrence of the issue in the logs (I checked for "AWS-UI CSS" in GitHub actions logs instead of looking at the test failures as we have retries. Before the fix these messages were consistently present).

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 97.31%. Comparing base (8c20e1c) to head (bfd5a47).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4256      +/-   ##
==========================================
+ Coverage   97.19%   97.31%   +0.11%     
==========================================
  Files         886      888       +2     
  Lines       26038    26029       -9     
  Branches     9438     9409      -29     
==========================================
+ Hits        25308    25329      +21     
+ Misses        724      694      -30     
  Partials        6        6              

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[pan-kot]

I renamed this file to "styles-check.test.tsx" as now there is a single util that we test.

[pan-kot]

I removed this file where the tests were done by calling checkMissingStyles and documentReadyAndIdle utils directly. Instead, all tests are now done against the useMissingStylesCheck.

I ensured that every test from this file is represented in the new tests.