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.

runtime/lua/vim/_meta/options.lua

@@ -5108,6 +5108,9 @@ vim.go.pb = vim.go.pumblend
 --- 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.
+---
 --- @type string
 vim.o.pumborder = ""
 vim.go.pumborder = vim.o.pumborder