feat(nats source): support end-to-end acknowledgements

[ryanbrooks-tomtom]

Summary

Adds opt-in end-to-end acknowledgement support for NATS JetStream messages. A
message is acknowledged only after downstream event finalization succeeds.

References

Related: #26311

Vector configuration

acknowledgements:
  enabled: true

sources:
  nats_in:
    type: nats
    url: nats://localhost:4222
    subject: events
    jetstream:
      stream: events
      consumer: vector
    acknowledgements:
      enabled: true

sinks:
  downstream:
    # Add sink-specific configuration.
    inputs:
      - nats_in
    acknowledgements:
      enabled: true

How did you test this PR?

• cargo vdev check fmt
• cargo vdev check rust
• make test SCOPE="nats"
• nats_jetstream_acks_delivered_message verifies successful downstream delivery clears JetStream ACK-pending state.
• nats_jetstream_does_not_ack_errored_delivery verifies errored events remain ACK-pending and are redelivered.
• nats_jetstream_extends_ack_wait_until_delivery verifies progress acknowledgements prevent redelivery during delayed finalization.
• nats_jetstream_keeps_ack_progress_while_output_is_blocked verifies downstream backpressure does not pause progress acknowledgements.
• nats_jetstream_requires_explicit_ack_policy verifies acknowledgement-enabled sources reject incompatible consumer policies.
• nats_core_preserves_valid_frames_after_decode_error verifies valid Core NATS frames are retained around recoverable decoding errors.

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. A changelog fragment is included.
• No.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[ryanbrooks-tomtom]

I have read the CLA Document and I hereby sign the CLA

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 6cd8c4df62

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: a4ab985090

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".