Prevent duplicate DLQ messages when afterDlq throws after successful publish

[Dimitris-Ilias]

Summary

As found on #75, on BaseQueueHandler.addToDLQ when an afterDlq hook throws after a successful DLQ publish a duplicate message is published on the DLQ.
This PR tackles this issue

Before this PR

BaseQueueHandler.addToDLQ wraps the decode → publish → afterDlq sequence in a single try/catch. The catch-path republishes the message as a raw-buffer fallback so that messages still reach the DLQ when something goes wrong.

• That fallback is correct when decode(msg) throws or when the first rabbit.publish is rejected by the broker — the DLQ entry is missing and the catch-path provides it.
• When afterDlq throws after a successful first publish, the catch-path runs anyway and publishes a second copy of the same message. The DLQ ends up with two entries for one input.
• Downstream consumers and DLQ replay tooling see duplicate work for every afterDlq failure under load.

After this PR

• addToDLQ tracks whether the first publish resolved by setting a local dlqPublished flag. The catch-path republishes only when !dlqPublished, so a successful publish + failing afterDlq produces exactly one DLQ entry.
• All three pre-existing failure modes still work correctly:
  • decode throws → fallback republish runs → 1 entry.
  • First rabbit.publish rejects → fallback republish runs → 1 entry.
  • afterDlq throws after success → fallback skipped → 1 entry (new behavior).
• Happy-path behavior is unchanged.
• Tests rewritten and extended to assert the new invariant via a real DLQ subscriber rather than just inspecting publish arguments.

[Copilot]

Pull request overview

Fixes a duplication bug in BaseQueueHandler.addToDLQ where a thrown afterDlq hook could cause the catch-path fallback publish to send a second (duplicate) DLQ message even after the initial DLQ publish succeeded.

Changes:

• Track whether the primary DLQ publish succeeded (dlqPublished) and skip fallback republish when it did.
• Update/extend tests to assert the “no duplicate DLQ entry” invariant and validate raw-buffer fallback behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
ts/base-queue-handler.ts	Adds a dlqPublished flag to prevent fallback republish when the first DLQ publish already succeeded.
test/base-queue-handler.test.ts	Reworks DLQ tests to assert single DLQ delivery and raw-buffer fallback on publish failure.

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[Dimitris-Ilias]

ℹ️ This is the new test but diff is a little out of whack because these two tests have very similar setup