Fix #2571: preserve context fields on scheduled-send wrap unwrap

[jeremydmiller]

Summary

Closes #2571. Alternative fix to PR #2572 — keeps that PR's two test files as the failing reproducers (which they were on plain main), but takes a different (smaller, hot-path-untouched) implementation path.

When an envelope is scheduled to a transport without native scheduled-send (RabbitMQ, Kafka, SharedMemory, …), MessageRoute.ForScheduledSend wraps it. The wrapper carried the right context fields but the inner — which is what eventually gets forwarded to the real destination — did not. End result: the cascaded timeout fires with TenantId=null and SagaId=null, multi-tenanted sagas miss their lookup, and observability fields drop.

Approach

Single-source-of-truth helper on Envelope, internal, six fields:

internal void CopyContextCorrelationFrom(Envelope source)
{
    CorrelationId  = source.CorrelationId;
    ConversationId = source.ConversationId;
    TenantId       = source.TenantId;
    UserName       = source.UserName;
    ParentId       = source.ParentId;
    SagaId         = source.SagaId;     // needed for the saga-timeout test
}

Two callers, both off the production hot path per the no-overhead constraint:

1. ScheduledSendEnvelopeHandler.HandleAsync — when the durable scheduler fires the wrapper back in, unwrap the inner and stamp it from the wrapper before forwarding via ForwardScheduledEnvelopeAsync (which deliberately does not re-stamp). The 5-line ad-hoc copy that was there in the WIP collapses into one helper call.

2. TrackedSession.ReplayAll — test-only replay path. No broker, no serialization, so unwrap + stamp + dispatch the inner here too. PlayScheduledMessagesAsync was the original culprit Jeremy pointed at: it dispatched wrapper.Message (the inner Envelope object) through a fresh InvokeAsync/SendAsync that built a new outgoing envelope with empty context. Now it unwraps to the inner, calls CopyContextCorrelationFrom, primes the replay bus's TenantId/CorrelationId/UserName so TrackEnvelopeCorrelation propagates them when building the freshly outgoing envelope (UserName isn't on DeliveryOptions, so the bus is the only path), then dispatches the inner's destination instead of the local-durable URI.

ForScheduledSend, MessageRoute, MessageBus.PublishAsync, TrackEnvelopeCorrelation, and the per-record tracker plumbing are all untouched.

Test plan

• CoreTests.Runtime.Scheduled.inner_envelope_is_stamped_before_serialization (in-process via SharedMemory + PlayScheduledMessagesAsync, asserts TenantId/CorrelationId/UserName) — passes
• Wolverine.RabbitMQ.Tests.scheduled_saga_timeout_preserves_tenant (Marten + RabbitMQ + durable outbox, end-to-end multi-tenant saga) — passes
• Full dotnet test src/Testing/CoreTests --framework net9.0 — 1364/1364 pass, no regression

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