[Fizz] Fix truncated prerender output under backpressure #37901

An aborted prerender can finish all tasks while partial output remains queued. If a Node stream applies backpressure, Fizz can close it before it flushes the remaining boundaries. This bug predates #37876 and also affects initial prerenders.

Fizz now waits for partialBoundaries to empty before it closes the destination. The regression test forces backpressure with highWaterMark: 1 and verifies that push() returns false. Without the fix, the stream emits the first boundary's partial content and closes before it emits the second's.