fix: Use border radius design token for column headers in sticky header table

[abdhalees]

Description

A more specific implementation of this reverted PR https://github.com/cloudscape-design/components/pull/1857

Related links, issue #, if available: n/a

How has this been tested?

Review checklist

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• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
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Testing

• Changes are covered with new/existing unit tests?
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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.36%. Comparing base (501edea) to head (4a5b497).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1867   +/-   ##
=======================================
  Coverage   95.36%   95.36%           
=======================================
  Files         690      690           
  Lines       18419    18419           
  Branches     5829     5829           
=======================================
  Hits        17565    17565           
  Misses        847      847           
  Partials        7        7           

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[jperals]

It would be nice to have more information on what this fixes from #1857. From the code it seems like it fixes the style of tables with header, but there is no new test that covers that case.

[abdhalees]

It would be nice to have more information on what this fixes from #1857. From the code it seems like it fixes the style of tables with header, but there is no new test that covers that case.

@jperals applying border radius on full-page variant caused the column headers bottom border to be shortend. this PR applies the border radius only on container and stacked variant and only top border radius, these are the case where the issue happens.

this was caught by a screenshot test so there is already an exiting test that failed.