[jeremydmiller]

Follow-up to #5107 / #5109 for the MySQL queue sender. Independent of #5109; each database engine gets its own PR.

The gap

MySqlQueueSender.SendAsync(Envelope) only took the atomic MoveFromOutgoingTo* path when Envelope.WasPersistedInOutbox was true. Envelopes stored by the EF Core / Marten outbox and envelopes resent by outbox recovery are not marked, so they were inserted into the queue on their own and the outbox row was deleted later, in a separate transaction by DurableSendingAgent. A node that dies between the two leaves the outbox row behind; outbox recovery resends it, and once the receiver's handled inbox row has expired (KeepAfterMessageHandling) the copy is handled a second time.

Fix

A durable send that isn't marked WasPersistedInOutbox now goes through MoveOrWriteToQueueAsync, which writes the queue (or scheduled) row and deletes the outbox row, if there is one, in one transaction. A client transaction; a duplicate is a no-op ON DUPLICATE KEY UPDATE id = id, so the delete still commits. (Not changed here: the existing marked MoveFromOutgoing* path runs its insert and delete as two autocommitted statements, so it isn't atomic either.) Marked envelopes keep the existing path; non-durable endpoints are unchanged. The DurableSendingAgent delete that follows becomes a no-op.

Tests

New Transport/Bug_5107_outbox_row_survives_the_queue_insert.cs:

• the queue insert removes the outbox row
• a scheduled send removes the outbox row
• an envelope with no outbox row still reaches the queue
• sending the same envelope twice leaves one queue row and no outbox row

Before the fix 3 of 4 failed ("outbox count should be 0 but was 1"); the no-outbox-row test was the control. After: 4/4. Locally on net9.0 the provider's Transport tests pass (60/60), and wolverine.slnx builds clean with -f net9.0.

Refs #5107

🤖 Generated with Claude Code

https://claude.ai/code/session_01DCggXgxnG3EBXoR9rrq37U