fix(cmdline): validate 'incsearch' cursor for "cmdline_show" redraw (#34630)

Problem:  "cmdline_show" event may be emitted with an invalid cursor
          position, causing a redraw that will clear the match highlight.
Solution: Mark the cursor position as valid so that a "cmdline_show"
          callback that updates the screen does not clear the match highlight.

src/nvim/ex_getln.c

@@ -556,6 +556,7 @@ static void may_do_incsearch_highlighting(int firstc, int count, incsearch_state
     curwin->w_cursor = s->search_start;
   } else if (found != 0) {
     curwin->w_cursor = end_pos;
+    curwin->w_valid_cursor = end_pos;  // mark as valid for cmdline_show redraw
   }
 
   msg_starthere();