fix: replay_bloc keeps more undo history than limit after limit is lowered #4901

Description

ReplayCubit/ReplayBloc (and the mixins) expose a limit setter: "Sets the internal undo/redo size limit." When limit is lowered after history has built up, the history is never trimmed to the new limit:

• Setting limit doesn't trim anything, so the next undo() calls go back further than the limit allows.
• After the next emit, _ChangeStack.add removes only one old entry (if, not while), so the history stays above the limit.

Steps To Reproduce

final cubit = CounterCubit(); // ReplayCubit<int>, no limit
cubit
  ..increment() // 1
  ..increment() // 2
  ..increment() // 3
  ..limit = 1
  ..undo()
  ..undo();

Expected: states 1, 2, 3, 2 (only one undo is allowed).
Actual: states 1, 2, 3, 2, 1.