Add shadow mode to NATS audit event streaming consumer

What does this MR do and why?

Introduce an ops-flagged shadow mode so the NATS consumer can run under real production load for validation and partition-count tuning without delivering to customer destinations. Shadow mode keeps all per-batch work (lookup, filtering, body building) but swaps the network stream_batch call for a simulated latency, then acks as success so live delivery is mirrored with no double-delivery.

Gated by ops flag audit_event_streaming_nats_shadow_mode, default off.

References

#604456

Screenshots or screen recordings

Before	After

How to set up and validate locally

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