fix: release gate reservation on retry to prevent queue wedge (#311)

[shimib]

What / why

Fixes #311. The Redis sorted-set flow's per-queue gate reservation (a LocalConcurrencyGate slot or a redis-quota INCR) was released only by resultWorker (terminal results). The retry path (retryWorker) re-enqueued the request without releasing, so every retry leaked a reservation:

• with LocalConcurrencyGate (no TTL), inFlight ratcheted up until Budget() hit 0 and the queue stopped dispatching permanently;
• with redis-quota, it was masked by the key's TTL reset — which itself over-admits while requests are in flight.

Compounded by activeReleases.Store overwriting a prior closure on re-dispatch, orphaning it.

Changes (pkg/redis/sortedset_impl.go)

• retryWorker now releases each retried request's reservation before re-enqueue (a re-dispatched request re-reserves via gate.Apply). Releasing before flushRetryBatch's ZAdd means a re-dispatch cannot race with / overwrite the reservation.
• Dispatch-time store uses sync.Map.Swap and releases any prior closure defensively — never orphans a lingering reservation.
• Regression test TestSortedSetFlow_RetryReleasesReservation for the retry→release path (previously untested — why the leak stayed latent). It fails without this fix and passes with it.

Not affected

Pub/Sub holds the reservation in a function-scoped defer that runs on every return path including Nack/retry — no analogous leak, so the fix is scoped to the sorted-set flow.

Testing

• make test green across all modules.
• New regression test verified to fail on main and pass with the fix.

Closes #311