Refactor inline video comment player styling ownership

[NotThatKindOfDrLiz]

Summary

The rounded-corner regression from PR #4706 was fixed in the right user-facing place, but the implementation left a small ownership issue behind. CommentItem became responsible for restoring the inline player's rounded corners, while VideoCommentPlayer remained unaware of that styling contract.

Why this should be cleaned up

• The visual contract belongs to the player component, not just one parent usage site.
• The regression test added in PR fix(comments): restore rounded corners on inline video player #4706 asserted a ClipRRect wrapper in CommentItem, which couples the test to one parent implementation detail instead of protecting the player API.
• Leaving the styling responsibility outside the player makes future reuse easier to get wrong and invites duplicate wrappers in other call sites.

Cleanup

• Add an optional border-radius API to VideoCommentPlayer.
• Pass the radius from CommentItem through that API instead of wrapping the player externally.
• Move the rounded-corner regression coverage to the player test, and leave CommentItem testing only the wiring.

Standard

This follow-up is needed to keep the no-new-tech-debt standard intact after PR #4706.