fix(go): back off before re-polling failed batch

[NikolayS]

Bug

In clients/go/consumer.go, the consumer loop slept pollInterval only on receive error or empty batch. When a batch WAS received but finalization failed — nackFailed true (loop did continue with no sleep) or Ack returned an error (fell through to the loop top with no sleep) — the loop re-polled immediately. Because the batch was never finished, pgque.next_batch returns the SAME batch instantly, so a persistent nack/ack failure (e.g. partial grants where receive works but nack/ack don't) produced a tight loop at full speed: re-receive same batch, re-run all handlers (duplicate side effects), fail again, repeat. Even one transient ack failure re-executed the whole batch's handlers with zero delay.

Fix

Sleep pollInterval (respecting ctx cancellation, same select { case <-ctx.Done() ... case <-time.After(...) } pattern used in the receive-error and empty paths) before re-polling on both the nack-failure path and the Ack-error path. The Ack n==0 stale/double-ack case stays warning-only and gains no sleep.

Red/green TDD: added TestConsumer_NackFailure_BacksOffBeforeRepoll and TestConsumer_AckFailure_BacksOffBeforeRepoll with a stub backend (redeliverStubBackend) whose Receive always returns the same batch and whose Nack/Ack fail; the tests bound the number of Receive calls in a 400 ms window with a 50 ms poll interval.

Verification

Red (on unfixed code, go test -run BacksOffBeforeRepoll ./... in clients/go):

--- FAIL: TestConsumer_NackFailure_BacksOffBeforeRepoll (0.45s)
    Receive called 52141 times in 400ms with pollInterval 50ms — tight re-poll loop on nack failure (want <= 11)
--- FAIL: TestConsumer_AckFailure_BacksOffBeforeRepoll (0.58s)
    Receive called 236297 times in 400ms with pollInterval 50ms — tight re-poll loop on ack failure (want <= 11)

Green (with fix, in clients/go):

$ go test ./...
ok  	github.com/NikolayS/pgque-go	4.784s
?   	github.com/NikolayS/pgque-go/bench/coop_demo	[no test files]
$ go vet ./...
(clean, no output)

Addresses finding B1 (Go) of #283

https://claude.ai/code/session_01KAaEGkQZmey1D1xCsVGmqv

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