fix(messages): no message kind for :write messages #31519

- Problem: cannot replace the initial bufwrite message (from `filemess`) by the final one (`"test.lua" [New] 0L, 0B written`), when using `vim.ui_attach`.
- Solution: add kind to both messages.

runtime/doc/ui.txt

@@ -789,6 +789,7 @@ must handle.
 	kind
 	    Name indicating the message kind:
 		"" (empty)	Unknown (consider a |feature-request|)
+		"bufwrite"	|:write| message
 		"confirm"	|confirm()| or |:confirm| dialog
 		"confirm_sub"	|:substitute| confirm dialog |:s_c|
 		"emsg"		Error (|errors|, internal error, |:throw|, …)

src/nvim/bufwrite.c

@@ -1148,6 +1148,7 @@ int buf_write(buf_T *buf, char *fname, char *sfname, linenr_T start, linenr_T en
     msg_scroll = true;              // don't overwrite previous file message
   }
   if (!filtering) {
+    msg_ext_set_kind("bufwrite");
     // show that we are busy
 #ifndef UNIX
     filemess(buf, sfname, "");
@@ -1763,6 +1764,7 @@ restore_backup:
     if (msg_add_fileformat(fileformat)) {
       insert_space = true;
     }
+    msg_ext_set_kind("bufwrite");
     msg_add_lines(insert_space, lnum, nchars);       // add line/char count
     if (!shortmess(SHM_WRITE)) {
       if (append) {

test/functional/ui/messages_spec.lua

@@ -1112,7 +1112,7 @@ stack traceback:
     command('write ' .. fname)
     screen:expect({
       messages = {
-        { content = { { string.format('"%s" [New] 0L, 0B written', fname) } }, kind = '' },
+        { content = { { string.format('"%s" [New] 0L, 0B written', fname) } }, kind = 'bufwrite' },
       },
     })
   end)