fix(mailbox): ignore duplicate inbound message ids

[0xC3B6]

Summary

• Make inbound mailbox writes idempotent when the same message id is received more than once.
• Keep the first stored inbound message and avoid appending duplicate entries to the pending inbound index.
• Add a regression test covering duplicate inbound ids.

Root cause

writeInbound() always appended the message id to _inbound, even when _messages already contained that id. A retried Hub delivery could therefore make poll() and countPending() report the same message more than once.

Testing

• node --test test/mailboxStore.test.js

[autogame-17]

Thanks @0xC3B6 — fantastic fix. We ported your change into our internal source tree and shipped it in v1.70.0-beta.5 (npm: @evomap/evolver@1.70.0-beta.5, Release: https://github.com/EvoMap/evolver/releases/tag/v1.70.0-beta.5).

Area: MailboxStore writeInbound idempotency on duplicate ids
Ported commit: 4a63e91

We work primarily in a private source-of-truth repo and publish an obfuscated mirror to this public repo, so we could not merge your branch here directly; your authorship is acknowledged in the release notes and commit trailer. Really appreciate the well-scoped PR and the regression test — that made adoption trivial.

Closing in favor of the shipped release.