fix(controls): Fix TopSeparator and FooterSeparator rendering in LeftNavigationView

[apachezy]

Pull request type

Please check the type of change your PR introduces:

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

• The Separator (Border) did not have a border brush set but had a border thickness > 0, which could make the border occupy space but remain invisible.
• The top and bottom border thickness of the Separator (Border) was 0.5 each, exactly filling the height of a 1-unit Border, preventing the background from showing.

Issue Number: N/A

What is the new behavior?

• The separators render correctly.

Other information

The issue seems to have originated from #1541, which attempted to fix #1514 but didn’t address the root cause.
This PR corrects the rendering logic to align with the intended design.
Note: #1514 has already been resolved by #1464.