fix(api): do not allow opening float to closing buffer

Problem: no check for nvim_open_win opening a new floating window into a closing
buffer, which can lead to crashes.

Solution: call check_split_disallowed; opening a new float semantically splits
from a window, so the same problems as regular splits apply. Also restore the
error if switch_win_noblock in win_set_buf fails (may not be possible to hit
this, but win_set_buf can silently succeed there since #31595).

As the lock check applies to curbuf (not the target buffer) this may feel a bit
restrictive, though this isn't different to how things like ":split" or
nvim_open_win with "split = true" works when targeting a different buffer. Only
checking the target buffer's lock will cause issues if win_set_buf doesn't end
up in the target buffer for whatever reason.

Maybe we could consider checking the lock of the buffer after win_set_buf and
close the window if it's locked (maybe a bit fiddly, especially as closing a
window can fail...), or make the open + switch operation more atomic, like how
Vim does for its popup windows..?

It also used to be the case that win_set_buf would set an error if autocommands
sent us to a different buffer than what was requested, but #31595 appears to
have also changed that... I haven't touched that here.

src/nvim/api/win_config.c

@@ -281,6 +281,9 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Err
       wp->w_config = fconfig;
     }
   } else {
+    if (!check_split_disallowed_err(curwin, err)) {
+      goto cleanup;  // error already set
+    }
     wp = win_new_float(NULL, false, fconfig, err);
   }
   if (!wp) {