Applied rounded corners to panel's contents view.
Introduced brave::GetPanelContentsRoundedCorners(PrefService*, bool has_header)
which derives corners from the pref and current header presence.
Route GetRoundedCorners() through it via patch, reading header_view_ state
(set before AddChildView) instead of the stale coordinator entry ID.
Call UpdateContentWrapperChildCorners() from UpdateBorder() overrides so
corners are re-applied on panel open and pref change.
Resolves no issue - use umbrella issue (brave/brave-browser#51462)
TEST=SidePanelUtilsTest.*, SidebarBrowserTest.SidebarV2ContentCornersUpdateOnStateChange
Simon Hong
9c51b5e6e2