fix(terminal): avoid mismatched busy_start without busy_stop (#32458)

Problem: `showmode` in `terminal_enter` may cause `vpeekc` to process events,
which may handle pending escape sequences. If `CSI ? 25 l` is handled to hide
the cursor, it may remain hidden even after leaving terminal mode if both
`terminal_enter` and (indirectly) `showmode` call `ui_busy_start`, as there is
only one matching call to `ui_busy_stop` after leaving terminal mode.

Solution: let `terminal_enter` handle setting the initial visibility of the
cursor before calling `showmode`.

Closes #32456.

This simple solution assumes it isn't possible for e.g. `os_breakcheck` to be
called indirectly by something else before `terminal_enter` initially handles
cursor visibility and after it restores it, which I think is true.

src/nvim/terminal.c

@@ -693,7 +693,6 @@ bool terminal_enter(void)
   refresh_cursor(s->term);
 
   adjust_topline(s->term, buf, 0);  // scroll to end
-  showmode();
   curwin->w_redr_status = true;  // For mode() in statusline. #8323
   redraw_custom_title_later();
   if (!s->term->cursor.visible) {
@@ -701,6 +700,7 @@ bool terminal_enter(void)
     ui_busy_start();
   }
   ui_cursor_shape();
+  showmode();
   apply_autocmds(EVENT_TERMENTER, NULL, NULL, false, curbuf);
   may_trigger_modechanged();