fix(ui): 'pumborder' parsing, shadow #36127

Problem:
1. Setting `pumborder=+,+,+,+,+,+,+,+` failed to render the custom
   border characters correctly. The issue occurred in `parse_winborder()`
   where it incorrectly used `p_winborder` instead of the `border_opt`
   parameter when the option value didn't contain commas.
2. In `pum_redraw()`, calling `parse_border_style()` directly with the
   option string failed to parse comma-separated border characters.
3. Missing documentation for PmenuShadow and PmenuShadowThrough
   highlight groups used by the shadow border style.
4. Coverity reports CID 631420: passing WinConfig (480 bytes) by value
   in `grid_draw_border()`.
5. crash when using `shadow` value on pumborder.

Solution:
1. Fix `parse_winborder()` to use `border_opt` parameter consistently,
   ensuring the correct option value is parsed regardless of which
   option (winborder/pumborder) is being set.
2. Update `pum_redraw()` to call `parse_winborder()` instead of
   `parse_border_style()`, properly handling both predefined styles
   and custom comma-separated border characters.
3. Add documentation for PmenuShadow (blended shadow areas) and
   PmenuShadowThrough (see-through corners) highlight groups.
4. Change `grid_draw_border()` to accept WinConfig by pointer.
5. When the "shadow" style is used, no additional row and column offset
   is applied, and the border width is reduced.

src/nvim/options.lua

@@ -6727,7 +6727,8 @@ local options = {
       values = { '', 'double', 'single', 'shadow', 'rounded', 'solid', 'bold', 'none' },
       desc = [=[
         Defines the default border style of popupmenu windows. Same as
-        'winborder'.
+        'winborder'. |hl-PmenuBorder| is used. When style is "shadow", the
+        |hl-PmenuShadow| and |hl-PmenuShadowThrough| are used.
       ]=],
       short_desc = N_('border of popupmenu'),
       type = 'string',