fix(api): on_detach consistently before buf_freeall autocmds (#35369)

Problem: on_detach may be called after buf_freeall and other important things,
plus its textlock restrictions are insufficient. This can cause issues such as
leaks, internal errors and crashes.

Solution: disable buffer updates in buf_freeall, before autocommands (like the
order after #35355 and when do_ecmd reloads a buffer). Don't do so in
free_buffer_stuff; it's not safe to run user code there, and buf_freeall already
runs before then; just free them to avoid leaks if buf_freeall autocommands
registered more for some reason.

(cherry picked from commit 2211953266)

Co-authored-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>

src/nvim/buffer.c

@@ -657,10 +657,6 @@ bool close_buffer(win_T *win, buf_T *buf, int action, bool abort_if_last, bool i
 
   buf->b_nwindows = nwindows;
 
-  // Disable buffer-updates for the current buffer.
-  // No need to check `unload_buf`: in that case the function returned above.
-  buf_updates_unload(buf, false);
-
   buf_freeall(buf, ((del_buf ? BFA_DEL : 0)
                     + (wipe_buf ? BFA_WIPE : 0)
                     + (ignore_abort ? BFA_IGNORE_ABORT : 0)));
@@ -789,6 +785,11 @@ void buf_freeall(buf_T *buf, int flags)
   bufref_T bufref;
   set_bufref(&bufref, buf);
 
+  buf_updates_unload(buf, false);
+  if (!bufref_valid(&bufref)) {
+    // on_detach callback deleted the buffer.
+    return;
+  }
   if ((buf->b_ml.ml_mfp != NULL)
       && apply_autocmds(EVENT_BUFUNLOAD, buf->b_fname, buf->b_fname, false, buf)
       && !bufref_valid(&bufref)) {
@@ -935,7 +936,7 @@ static void free_buffer_stuff(buf_T *buf, int free_flags)
   map_clear_mode(buf, MAP_ALL_MODES, true, true);   // clear local abbrevs
   XFREE_CLEAR(buf->b_start_fenc);
 
-  buf_updates_unload(buf, false);
+  buf_free_callbacks(buf);
 }
 
 /// Go to another buffer.  Handles the result of the ATTENTION dialog.