Don't start an implicit EF Core transaction on ScheduleAsync for a Wolverine-mapped DbContext (closes #3121)

[jeremydmiller]

Problem

For a Wolverine-mapped DbContext, IDbContextOutbox.ScheduleAsync starts an explicit EF Core transaction the caller never asked for, while PublishAsync does not:

outbox.Enroll(db);
await outbox.PublishAsync(new TestMsg());   // no implicit EF transaction
await outbox.ScheduleAsync(new TestMsg(), TimeSpan.FromMinutes(1));  // starts one ❌

Root cause

ScheduleAsync produces an envelope with EnvelopeStatus.Scheduled, which IEnvelopeTransaction.PersistAsync routes to PersistIncomingAsync (IEnvelopeTransaction.cs:33-34). EfCoreEnvelopeTransaction.PersistIncomingAsync began an explicit transaction unconditionally, before checking IsWolverineEnabled(). But for a mapped DbContext the envelope is simply tracked via DbContext.Add(new IncomingMessage(...)) and flushed inside SaveChangesAsync's own implicit transaction — the explicit one is pure overhead.

This is an oversight from a prior refactor, not a deliberate tradeoff. Commit 5af99ae93 ("Outbox behavior alignment with EF Core transaction model") moved the unconditional BeginTransactionAsync out of both PersistOutgoingAsync overloads into their raw (non-mapped) branch — but left PersistIncomingAsync untouched. That's the entire asymmetry the reporter sees.

Fix

Apply the same alignment to PersistIncomingAsync: only begin an explicit transaction in the raw / non-IsWolverineEnabled branch (where an ADO command must share the caller's transaction). The mapped branch just tracks the entity, exactly like PersistOutgoingAsync. One method; no API change.

The two other callers are unaffected:

• TryMakeEagerIdempotencyCheckAsync begins its own explicit transaction (EfCoreEnvelopeTransaction.cs:140-143).
• CommitAsync's handled-idempotency insert already relied on a later SaveChanges to flush the tracked Add, not on this forced transaction.

Tests

• New regression test persisting_against_mapped_dbcontext_does_not_start_an_explicit_transaction — asserts that persisting a scheduled/incoming and an outgoing envelope against a Wolverine-mapped DbContext leaves Database.CurrentTransaction null.
• The full end_to_end_efcore_persistence class + eager-idempotency tests (23 total) remain green, confirming the raw-mode persistence and idempotency paths are unaffected.

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