fix(logstash source): never emit partial ACKs

[bruceg]

Summary

Change the ACK generation algorithm to only ever ACK on completed windows. While partial ACKs are permitted by the official protocol spec and cause no problems for the go-lumber client in Beats, it appears to confuse proxies that assume there will only be one ACK per window, causing errors on subsequent batches.

This requires a separate change to prevent ACKing a WindowSize frame that arrives before the current window has received all its advertised events and instead fail it as a fatal decode error, closing the connection. Again, this is allowed by the official protocol specification, real clients (go-lumber, beats) always send exactly window_size events per window. More importantly, the reference go-lumber server treats a mid-window WindowSize as a protocol error. Matching actual implementations is more important than following the spec.

Note that the two changes are in separate commits in order to facilitate review.

Vector configuration

Tested with both a filebeat client sending directly to Vector with a combination of delay processors and the blackhole sink, as well as a production pipeline with live multiplexed connections.

How did you test this PR?

As above, with the new conditions encoded in unit tests.

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Related: #23346 #25531 #25655 #25664

Notes

• Please read our Vector contributor resources.
• Do not hesitate to use @vectordotdev/vector to reach out to us regarding this PR.
• Some CI checks run only after we manually approve them.
  • We recommend adding a pre-push hook, please see this template.
  • Alternatively, we recommend running the following locally before pushing to the remote branch:
    • make fmt
    • make check-clippy (if there are failures it's possible some of them can be fixed with make clippy-fix)
    • make test
• After a review is requested, please avoid force pushes to help us review incrementally.
  • Feel free to push as many commits as you want. They will be squashed into one before merging.
  • For example, you can run git merge origin master and git push.
• If this PR introduces changes Vector dependencies (modifies Cargo.lock), please
  run make build-licenses to regenerate the license inventory and commit the changes (if any). More details on the dd-rust-license-tool.