fix(forms): ensure `Message` validation icon has a role of "img"

Description

This fixes an accessibility issue to ensure the Message icon's aria-label is available to screen readers.

[ ] :ok_hand: ~design updates will be Garden Designer approved (add the designer as a reviewer)~
[ ] :globe_with_meridians: ~demo is up-to-date (npm start)~
[ ] :arrow_left: ~renders as expected with reversed (RTL) direction~
[ ] :metal: ~renders as expected with Bedrock CSS (?bedrock)~
[x] :guardsman: includes new unit tests. Maintain existing coverage (always >= 96%)
[x] :wheelchair: tested for WCAG 2.1 AA accessibility compliance
[x] :memo: tested in Chrome, Firefox, Safari, and Edge