fix(outbox): discard clears incoming envelope after rolled-back outbox

[robertdusek]

Problem

When a durable-local-queue handler enlists the Marten (or Polecat) outbox and SaveChangesAsync rolls back — e.g. a document insert hits a unique constraint — an .OnException<T>().Discard() policy fails to remove the incoming envelope. It stays status='Incoming' in wolverine_incoming_envelopes and is reprocessed on every InboxStaleTime reclaim / restart.

Root cause

FlushOutgoingMessagesOnCommit.BeforeSaveChangesAsync flips Envelope.Status = Handled in memory before the commit, alongside queuing the mark-handled UPDATE into the same batch. On rollback the DB row stays Incoming, but the in-memory flag is not reverted. The discard path then hits the _markAsHandled optimization in DurableReceiver (if (e.Status == Handled) return;), which trusts the stale flag and skips the real UPDATE — so the row is never marked handled.

Fix

Defer the in-memory Status = Handled flip to AfterCommitAsync, which only runs once the commit is durable. Polecat's ITransactionParticipant path has no after-commit hook, so it simply stops setting the premature flag — the idempotent _markAsHandled UPDATE covers the success path.

Testing

• New red-green regression tests for Marten and Polecat (both fail before the fix with Incoming == 1, pass after; verified against the reverted code on both stores).