fix(api): win_set_config autocmds crash when moving win to other tabpage

Problem: win_enter autocommands can close new_curwin, crashing if it was the
last window in its tabpage after removing win, or can close parent, crashing
when attempting to split it later.

Solution: remove win first, check that parent is valid after win_enter.

NOTE: This isn't actually quite right, as this means win is not in the window
list or even has a frame when triggering enter autocommands (so it's not
considered valid in the tabpage). This is addressed in later commits.

src/nvim/api/win_config.c

@@ -478,12 +478,17 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
         int dir;
         new_curwin = winframe_remove(win, &dir, win_tp == curtab ? NULL : win_tp);
       }
+      win_remove(win, win_tp == curtab ? NULL : win_tp);
       // move to neighboring window if we're moving the current window to a new tabpage
       if (curwin == win && parent != NULL && new_curwin != NULL
           && win_tp != win_find_tabpage(parent)) {
         win_enter(new_curwin, true);
+        if (!win_valid_any_tab(parent)) {
+          // win_enter autocommands closed the `parent` to split from.
+          api_set_error(err, kErrorTypeException, "Window to split was closed");
+          return;
+        }
       }
-      win_remove(win, win_tp == curtab ? NULL : win_tp);
     } else {
       win_remove(win, win_tp == curtab ? NULL : win_tp);
       ui_comp_remove_grid(&win->w_grid_alloc);

test/functional/api/window_spec.lua

@@ -1663,6 +1663,27 @@ describe('API/win', function()
         },
       }, fn.winlayout())
     end)
+
+    it('closing new curwin when moving window to other tabpage works', function()
+      command('split | tabnew')
+      local w = api.nvim_get_current_win()
+      local t = api.nvim_get_current_tabpage()
+      command('tabfirst | autocmd WinEnter * ++once quit')
+      api.nvim_win_set_config(0, { win = w, split = 'left' })
+      -- New tabpage is now the only one, as WinEnter closed the new curwin in the original.
+      eq(t, api.nvim_get_current_tabpage())
+      eq({ t }, api.nvim_list_tabpages())
+    end)
+
+    it('closing split parent when moving window to other tabpage aborts', function()
+      command('split | tabnew')
+      local w = api.nvim_get_current_win()
+      command('tabfirst | autocmd WinEnter * call nvim_win_close(' .. w .. ', 1)')
+      eq(
+        'Window to split was closed',
+        pcall_err(api.nvim_win_set_config, 0, { win = w, split = 'left' })
+      )
+    end)
   end)
 
   describe('get_config', function()