Fix: ThrottleFirstLastFrame emits default(T) after a trailing value was published

[gajk1034]

_ThrottleFirstLastFrame.MoveNext clears lastValue on window close but does not reset hasValue, unlike the TimeSpan-based _ThrottleFirstLast variants which reset both. Once a window has published a trailing value, hasValue stays true forever, so every subsequent window that received only one value re-emits the already-cleared lastValue — i.e. default(T) — when it closes.

  var frameProvider = new FakeFrameProvider();
  var subject = new Subject<int>();
  var results = new List<int>();

  subject.ThrottleFirstLastFrame(5, frameProvider).Subscribe(results.Add);

  subject.OnNext(1);          // opens window, emits 1                                                                                                                           
  subject.OnNext(2);          // stored as trailing value                                                                                                                        
  frameProvider.Advance(5);   // window closes, emits 2 (OK)                                                                                                                     

  subject.OnNext(3);          // opens a new window, emits 3                                                                                                                     
  frameProvider.Advance(5);   // window closes, emits 0 = default(int)                                                                                                           

  // results: [1, 2, 3, 0] — expected [1, 2, 3] 

This PR resets hasValue on window close, matching the TimeSpan-based implementation, and adds a regression test.

The added test fails on current main with [1, 2, 3, 0] (an extra default(int) emission) and passes with this fix.