IsAllRateLimited() short-circuited true for any pool with 0 or 1
keys, with minRetrySeconds left at 0 regardless of whether that key
was actually rate-limited. SleepUntilAnyAvailable() checks
"minRetry <= 0" to decide whether to skip sleeping, so with exactly
one key it always skipped the wait and let callers hammer the API
again immediately after a 429, even mid-cooldown.
The per-key loop already computes this correctly for any key count;
the special case only needs to cover the true no-keys edge case,
where there's nothing to wait on.
jim
f541ec4260