oncetask: WithPollInterval option for consumer-loop poll cadence

[anish749]

Problem

The consumer-loop idle-poll interval was hardcoded to 1 minute (oncetask/once_task_firestore.go). That puts a 1-minute floor on how late a time-readied task (waitUntil expiry, retry backoff, recurrence occurrence) can fire relative to its scheduled time. Workloads that need lower latency had no knob.

Solution

Add WithPollInterval(d time.Duration) HandlerOption. Stored on handlerConfig.PollInterval, defaults to 1 minute, non-positive values ignored. The consumer loop reads the value once at start when constructing its ticker.

Context

Per-handler option, same pattern as WithLeaseDuration / WithConcurrency. The value is read once at runLoop start — re-registration won't reset a live ticker, which matches how Concurrency already behaves (workers are spawned at registration). New-task creation continues to wake the loop immediately via evaluateChan; the poll interval only governs the time-based fallback.