fix(api): patch some cmdwin/textlock holes

Problem: there are new ways to escape textlock or break the cmdwin in
nvim_win_set_config and nvim_tabpage_set_win.

Solution: fix them. Use win_goto to check it in nvim_tabpage_set_win and use the
try_start/end pattern like with similar functions such as nvim_set_current_win
(which uses the existing msg_list, if set).

Careful not to use `wp->handle` when printing the window ID in the error message
for nvim_tabpage_set_win, as win_goto autocommands may have freed the window.

On a related note, I have a feeling some API functions ought to be checking
curbuf_locked...

src/nvim/api/tabpage.c

@@ -146,7 +146,11 @@ void nvim_tabpage_set_win(Tabpage tabpage, Window win, Error *err)
   }
 
   if (tp == curtab) {
-    win_enter(wp, true);
+    try_start();
+    win_goto(wp);
+    if (!try_end(err) && curwin != wp) {
+      api_set_error(err, kErrorTypeException, "Failed to switch to window %d", win);
+    }
   } else if (tp->tp_curwin != wp) {
     tp->tp_prevwin = tp->tp_curwin;
     tp->tp_curwin = wp;

src/nvim/api/win_config.c

@@ -451,6 +451,12 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
     if (!check_split_disallowed_err(win, err)) {
       return;  // error already set
     }
+    // Can't move the cmdwin or its old curwin to a different tabpage.
+    if ((win == cmdwin_win || win == cmdwin_old_curwin) && parent != NULL
+        && win_find_tabpage(parent) != win_tp) {
+      api_set_error(err, kErrorTypeException, "%s", e_cmdwin);
+      return;
+    }
 
     bool to_split_ok = false;
     // If we are moving curwin to another tabpage, switch windows *before* we remove it from the

src/nvim/winfloat.c

@@ -55,6 +55,19 @@ win_T *win_new_float(win_T *wp, bool last, WinConfig fconfig, Error *err)
       api_set_error(err, kErrorTypeException,
                     "Cannot change window from different tabpage into float");
       return NULL;
+    } else if (cmdwin_win != NULL && !cmdwin_win->w_floating) {
+      // cmdwin can't become the only non-float. Check for others.
+      bool other_nonfloat = false;
+      for (win_T *wp2 = firstwin; wp2 != NULL && !wp2->w_floating; wp2 = wp2->w_next) {
+        if (wp2 != wp && wp2 != cmdwin_win) {
+          other_nonfloat = true;
+          break;
+        }
+      }
+      if (!other_nonfloat) {
+        api_set_error(err, kErrorTypeException, "%s", e_cmdwin);
+        return NULL;
+      }
     }
     int dir;
     winframe_remove(wp, &dir, NULL, NULL);