Fix broken sliding window logic and remove unnecessary finisher in SlidingWindowGatherer

[simpanda01]

Fixes #18683

Problem:

• The integrator pushed the window before checking its size and unconditionally removed the first element after every push, so every emitted window had size 1 instead of forming an actual sliding window.
• The finisher() override was a no-op, duplicating the default behavior already provided by Gatherer.

Fix:

• Added a WINDOW_SIZE constant. Now elements are removed from the front only once the window exceeds WINDOW_SIZE, and a window is pushed downstream only once it reaches WINDOW_SIZE.
• Removed the redundant finisher() override and unused BiConsumer import.
• Kept ArrayDeque for O(1) removeFirst().

Testing: Verified with input 1,2,3,4,5 → correctly produces [1,2,3], [2,3,4], [3,4,5].