fix: Extend clickable area of Prompt Input

[connorlanigan]

Description

In the Prompt Input component, the actual textarea element is much smaller than the component's area when there are secondary actions. The empty area between the secondary actions and the submit button visually appears to be part of the text area, but clicking it de-focuses the textarea.

This code change makes it so that the empty area can be clicked as if it was part of the actual textarea.

Related links, issue #, if available: AWSUI-60362

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?

---

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 96.42%. Comparing base (005def6) to head (2b381d2).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3299   +/-   ##
=======================================
  Coverage   96.42%   96.42%           
=======================================
  Files         791      791           
  Lines       22580    22583    +3     
  Branches     7794     7804   +10     
=======================================
+ Hits        21772    21775    +3     
+ Misses        801      754   -47     
- Partials        7       54   +47     

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