chore(support-prompt-group): use borderRadiusSupportPrompt token for corner radius

[gethinwebster]

The support prompt group's corner radius uses the dedicated borderRadiusSupportPrompt design token (awsui.$border-radius-support-prompt) for all four corners.

Dependency: requires borderRadiusSupportPrompt to be published in @cloudscape-design/design-tokens (cloudscape-design/components#4923). Kept as a draft until that token lands and publishes — the build will be red until the token is available in the consumed design-tokens version.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.79%. Comparing base (47881c0) to head (a77792c).

❌ Your project check has failed because the head coverage (87.79%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #165   +/-   ##
=======================================
  Coverage   87.79%   87.79%           
=======================================
  Files          25       25           
  Lines         344      344           
  Branches       92       92           
=======================================
  Hits          302      302           
  Misses         42       42           

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