@ConcurrencyLimit(limit=2, policy=BLOCK), 4 concurrent callers, each sleeps 300ms
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per-caller completion time (ms), sorted: [300, 300, 598, 600]
total wall time for all 4 callers: 601ms
all 4 calls succeeded (BLOCK never rejects): true
expectation: with limit=2 and 300ms per call, 4 callers must take roughly 2x300=600ms+,
not ~300ms as they would with no limit at all.