Flaky integration spec: streams_pool_publisher_autoscale expects peak > 3, gets 3 under slow runners

[mhenrixon]

Seen on PR #399 run 31612294828, Integration (PG 18), seed 43887 — commit touched only pgmq schema-install framing, nowhere near the streams pool; the same job passed on the PR's previous commit.

1) Streams pool publisher autoscale grows the streams pool from the publish path under a concurrent broadcast storm
   Failure/Error: expect(peak).to be > 3           # the publish path grew the pool into headroom
     expected: > 3
          got:   3
   # ./spec/integration/streams_pool_publisher_autoscale_spec.rb:74

Load-sensitive assertion: on a saturated CI runner the broadcast storm may not generate enough concurrent demand to grow the pool past its floor within the window. Needs either a demand-generating synchronization (hold N publishers in-flight simultaneously before measuring peak) or a longer/bounded-growth window — not a blind threshold on scheduler behavior.