fix(display): wrong cursor column with 'concealcursor' = "n" and virt_text (#33218)

Problem:  Inline virtual text placed in a decor provider callback
          invalidates `w_virtcol`, which must be valid for `win_line()`.
Solution: Call `validate_virtcol()` after "line" decor provider callbacks.

src/nvim/drawline.c

@@ -1163,7 +1163,8 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, b
       }
     }
 
-    decor_providers_invoke_line(wp, lnum - 1);
+    decor_providers_invoke_line(wp, lnum - 1);  // may invalidate wp->w_virtcol
+    validate_virtcol(wp);
 
     has_decor = decor_redraw_line(wp, lnum - 1, &decor_state);