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.

runtime/doc/news.txt

@@ -219,6 +219,7 @@ HIGHLIGHTS
 • |hl-DiffTextAdd| highlights added text within a changed line.
 • |hl-OkMsg| |hl-StderrMsg| |hl-StdoutMsg|
 • |hl-SnippetTabstopActive| highlights the currently active tabstop.
+• |hl-PmenuBorder |hl-PmenuShadow| |hl-PmenuShadowThrough| see 'pumborder'.
 
 LSP
 
@@ -301,6 +302,7 @@ OPTIONS
 • 'winborder' "bold" style, custom border style.
 • |g:clipboard| accepts a string name to force any builtin clipboard tool.
 • 'busy' sets a buffer "busy" status. Indicated in the default statusline.
+• 'pumborder' add a border to the popup menu.
 
 PERFORMANCE
 

runtime/doc/options.txt

@@ -4912,7 +4912,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 'pumborder'		string	(default "")
 			global
 	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.
 
 						*'pumheight'* *'ph'*
 'pumheight' 'ph'	number	(default 0)

runtime/doc/syntax.txt

@@ -5354,6 +5354,11 @@ Normal		Normal text.
 NormalFloat	Normal text in floating windows.
 							*hl-FloatBorder*
 FloatBorder	Border of floating windows.
+							*hl-FloatShadow*
+FloatShadow	Blended areas when border is shadow.
+							*hl-FLoatShadowThrough*
+FloatShadownThrough
+		shadow corners when border is shadow.
 							*hl-FloatTitle*
 FloatTitle	Title of floating windows.
 							*hl-FloatFooter*
@@ -5382,6 +5387,13 @@ PmenuMatch	Popup menu: Matched text in normal item. Combined with
 							*hl-PmenuMatchSel*
 PmenuMatchSel	Popup menu: Matched text in selected item. Combined with
 		|hl-PmenuMatch| and |hl-PmenuSel|.
+							*hl-PmenuBorder*
+PmenuBorder	Popup menu: border of popup menu.
+							*hl-PmenuShadow*
+PmenuShadow	Popup menu: blended areas when 'pumborder' is shadow.
+							*hl-PmenuShadowThrough*
+PmenuShadownThrough
+		Popup menu: shadow corners when 'pumborder' is shadow.
 							*hl-ComplMatchIns*
 ComplMatchIns	Matched text of the currently inserted completion.
 							*hl-PreInsert*

runtime/lua/vim/_meta/options.lua

@@ -5130,7 +5130,8 @@ vim.go.pumblend = vim.o.pumblend
 vim.go.pb = vim.go.pumblend
 
 --- 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.
 ---
 --- @type string
 vim.o.pumborder = ""