fix: Add characterCountText to form field for improved screen reader support

[avinashbot]

Description

Apparently, NVDA/JAWS don't fare well when you update the accessible description of an input you're focused on after every keypress, and end up interrupting everything else, like the keypress announcements themselves.

This introduces a new string property, characterCountText, whose updates are debounced for screen readers. This also means that only the updated section of the constraint text ("Character count: x/y") is announced when it changes.

Related links, issue #, if available: AWSUI-61742

How has this been tested?

Added unit tests and a test util method.

Provisioning a Windows machine right now, but I'll also let our accessibility specialists take a crack at it before merging. But in the meantime, the code itself is reviewable.

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?

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.44%. Comparing base (f7f12ee) to head (ea583aa).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4332      +/-   ##
==========================================
+ Coverage   97.43%   97.44%   +0.01%     
==========================================
  Files         897      897              
  Lines       26342    26412      +70     
  Branches     9517     9531      +14     
==========================================
+ Hits        25667    25738      +71     
- Misses        632      668      +36     
+ Partials       43        6      -37     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[avinashbot]

Moved back to draft to test out a LiveRegion-based alternative, but it was messy and didn't work out very well. The current option isn't great either, because the announcement of the actually changed text (Character count: x/y) is frontloaded by (potentially) multiple sentences of completely static aria-describedby content. But this is the best we can do — this entire PR only exists to work around a serious bug with NVDA/JAWS.

@ernst-dev So not much actually changed since your last review (other than addressing your naming comment, which was a great pick)