Flux backpressure proof: /stream, requested in batches of 3, 2, then 45
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StepVerifier.create(controller.streamResults(), 3)
  .expectNext("event-0", "event-1", "event-2")
  .expectNoEvent(Duration.ofMillis(80))   -- no 4th item arrives without a request
  .thenRequest(2).expectNext("event-3", "event-4")
  .thenRequest(45).expectNextCount(45)
  .expectComplete()

RESULT: verified -- the flux emitted exactly as many items as were requested, in
the order requested, with no items arriving ahead of a pending request. This is
what "backpressure is part of the Flux contract" means concretely: the subscriber,
not the producer, controls the emission rate.