fix(window): yeet grid later when closing float in other tabpage #40758

Problem: #40731 may still crash if close_buffer autocmds reinsert the float's
grid. Plus removing the grid (and posting win_close) is unneeded if
win_close_othertab refuses to close the window later, which is possible.

Solution: do the stuff before freeing the window, like win_close.
(cherry picked from commit 1741da8412)

test/functional/ui/float_spec.lua

@@ -10460,12 +10460,22 @@ describe('float window', function()
     it('no crash when closing a floating window from a non-current tab', function()
       local buf = api.nvim_create_buf(false, true)
       local win = api.nvim_open_win(buf, false, { relative = 'editor', width = 5, height = 5, row = 0, col = 0 })
-      exec_lua(function()
+      local triggered = exec_lua(function()
         vim.cmd.tabnew()
         vim.api.nvim_win_call(win, vim.cmd.redraw)
+        local triggered = false
+        vim.api.nvim_create_autocmd('BufHidden', {
+          once = true,
+          buf = buf,
+          callback = function()
+            vim.api.nvim_win_call(win, vim.cmd.redraw)
+            triggered = true
+          end,
+        })
+        vim.api.nvim_win_close(win, true)
+        return triggered
       end)
-      api.nvim_win_close(win, true)
-      assert_alive()
+      eq(true, triggered)
     end)
 
     it(':sleep cursor placement #22639', function()