Fix ListBox HeaderHeight below the 32-pixel default

Setting a ListBox HeaderHeight below the 32-pixel default does not fully apply. The header band itself shrinks, but the Graphics object passed to PaintHeaderBackground still reports the default 32-pixel height instead of the configured header height, so custom header painting draws at the wrong size. The vertical separator lines between header columns are also drawn 32 pixels tall and extend below the shortened header into the first row.

Expected: with HeaderHeight set to 20, PaintHeaderBackground receives a Graphics sized to the 20-pixel header, and the header separators stay within the header band.

User impact: any ListBox with a header shorter than 32 pixels gets mis-sized custom header painting and separator lines that bleed into the first data row.