fix(ui): apply 'pumborder' to mouse menu, fix overflow #36193

Problem:
Mouse popup menus (right-click context menus) do not respect the
'pumborder' option and could overflow screen boundaries when borders
were enabled near the edge.

Solution:
- Remove the mouse menu exclusion from border rendering.
- Add boundary check to shift menu left when border would exceed screen
  width, ensuring complete visibility of menu content and borders.

src/nvim/options.lua

@@ -6699,6 +6699,9 @@ local options = {
         Defines the default border style of popupmenu windows. See 'winborder' for
         valid values. |hl-PmenuBorder| is used for highlighting the border, and when
         style is "shadow" the |hl-PmenuShadow| and |hl-PmenuShadowThrough| groups are used.
+
+        This option also applies to mouse popup menus when 'mousemodel' is set to
+        "popup" or "popup_setpos", which will display borders using the same style.
       ]=],
       short_desc = N_('border of popupmenu'),
       type = 'string',