enhancement(acknowledgements): support best-effort sink branches

[Jansen-w]

Summary

This draft explores selective sink participation in end-to-end acknowledgements. A sink with acknowledgements.enabled: false becomes a best-effort branch: it still receives events, but its event copies do not retain source finalizers and therefore cannot delay or affect the upstream acknowledgement result.

The implementation:

• computes required and explicitly best-effort reachability during configuration compilation
• carries the resulting requirement on fanout connections
• removes source finalizers from best-effort event copies before they enter sink buffers
• preserves existing behavior for sinks whose acknowledgement setting is unspecified
• treats tap outputs as best effort

Known prototype limitations

• acknowledgement requirement changes during topology reload do not yet rebuild affected sources or reconnect every unchanged upstream edge
• best-effort branches still participate in fanout backpressure while their buffers accept events
• memory and disk buffer lifecycle coverage needs to be expanded before this is ready for production

References

None.

Vector configuration

sources:
  input:
    type: http_server
    address: 0.0.0.0:8080

sinks:
  required:
    type: http
    inputs: [input]
    uri: https://required.example.com/events
    acknowledgements:
      enabled: true
    encoding:
      codec: json

  best_effort:
    type: http
    inputs: [input]
    uri: https://best-effort.example.com/events
    acknowledgements:
      enabled: false
    encoding:
      codec: json

How did you test this PR?

• cargo test -p vector-core fanout::tests --lib
• cargo test -p vector --features sources-file,sinks-file config::acknowledgements_tests::propagates_settings --lib
• cargo test -p vector --features sources-http_server,sinks-http topology::test::end_to_end --lib
• cargo check -p vector --lib
• cargo clippy -p vector-core --tests -- -D warnings
• cargo clippy -p vector --lib --features sources-http_server,sinks-http,sources-file,sinks-file -- -D warnings
• cargo fmt --all -- --check
• cargo vdev check changelog-fragments

The end-to-end test blocks a failing best-effort HTTP sink and verifies that the HTTP source responds successfully as soon as the required sink succeeds.

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

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