fix(nats source): defer JetStream acks until delivery

[horjulf]

Summary

The JetStream NATS source currently acknowledges a broker message immediately after send_batch hands decoded events to the internal topology. That remains ahead of actual sink delivery even when end-to-end acknowledgements are enabled, so a Vector crash can lose an already-ACKed message that was still buffered downstream.

This draft makes JetStream participate in the existing batch-finalizer framework:

• attach one BatchNotifier to all events decoded from a NATS message;
• ACK the message only after all connected acknowledgement-enabled sinks report Delivered;
• NAK Errored or Rejected batches so JetStream redelivers them;
• preserve the existing immediate-ACK behavior when end-to-end acknowledgements are disabled;
• advertise acknowledgement support only in JetStream mode; Core NATS remains unchanged.

The source requires an explicit JetStream consumer acknowledgement policy when end-to-end acknowledgements are active. An unordered downstream completion paired with cumulative all acknowledgements could otherwise acknowledge an earlier failed message.

References

No existing NATS acknowledgement issue found. Opening this as a draft for design feedback, particularly around NAK pacing and whether periodic progress ACKs should be added for delivery times longer than the consumer ack_wait.

Vector configuration

acknowledgements:
  enabled: true

sources:
  queued_logs:
    type: nats
    url: nats://localhost:4222
    subject: logs.vl
    jetstream:
      stream: fleet-logs
      consumer: victorialogs # configured with ack_policy: explicit
    decoding:
      codec: native

sinks:
  output:
    type: console
    inputs: [queued_logs]
    acknowledgements:
      enabled: true

How did you test this PR?

• make check-clippy — all workspace targets and features.
• make test FEATURES=nats-integration-tests SCOPE=nats_jetstream_ against a local NATS JetStream 2.14.5 server — 7/7 tests passed.
• Added a broker-backed test proving a message remains pending until downstream finalization, rejection triggers redelivery, and successful delivery advances the ACK state.
• Added a broker-backed test rejecting cumulative consumer acknowledgement policy when end-to-end acknowledgements are enabled.
• make check-fmt.
• make check-markdown.
• make check-changelog-fragments.
• make check-generated-docs — 248 component examples validated.

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. A changelog fragment and generated NATS source documentation are included.
• No. A maintainer will apply the no-changelog label to this PR.

AI assistance: OpenAI GPT-5.6 Codex was used to help implement and validate this change. The resulting behavior was verified against the source acknowledgement framework and a real JetStream server.

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[horjulf]

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