Preserve correlation/trace context across Global Partitioning interceptor (supersedes #2709)

[jeremydmiller]

Supersedes #2709 — same problem, same test contract, alternative implementation.

Why a new PR

The original PR's test surface is a strict improvement and Lyall's underlying analysis is correct: GlobalPartitionedInterceptor previously built a fresh MessageBus(_runtime) with no inbound envelope, so PropagateHeadersRule.ApplyCorrelation short-circuited (originator.Envelope was null) and the re-published envelope dropped correlation, tenant id, and distributed-trace continuity. Allowlists declared via PropagateIncomingHeadersToOutgoing were silently ignored.

The implementation here keeps every behavioural improvement from #2709 but routes the field copying through the existing Envelope.CopyContextCorrelationFrom helper rather than re-stating the field list inline.

What changed vs #2709

#2709 copies context fields via two distinct mechanisms:

1. RouteBus constructor manually copies CorrelationId / TenantId / UserName from the inbound onto the bus.
2. DeliveryOptions carries CorrelationId / TenantId / GroupId into the routing call.

This PR collapses both into a single override of MessageBus.TrackEnvelopeCorrelation:

internal override void TrackEnvelopeCorrelation(Envelope outbound, Activity? activity)
{
    if (outbound.Source.IsEmpty())
        outbound.Source = Runtime.Options.ServiceName;

    outbound.CopyContextCorrelationFrom(_inbound);
    outbound.Store = Storage;
}

Envelope.CopyContextCorrelationFrom (added in GH-2571 / PR #2572) is the canonical helper for "this envelope is forwarding another's context". The other two callers — ScheduledSendEnvelopeHandler (durable scheduled-send unwrap) and TrackedSession.ReplayAll (in-memory tracked-session replay) — are both forwarding shapes; the global-partitioning re-route is a third instance of exactly the same shape (same logical message, different shard) so it should reuse the same helper.

The benefit is that the helper covers the full context-correlation set in one call:

Field	base bus tracking	CopyContextCorrelationFrom
CorrelationId	from bus property	from inbound
ConversationId	reset to outbound's own Id	from inbound
TenantId	from bus property	from inbound
UserName	from bus property	from inbound
ParentId	from Activity.Current	from inbound
SagaId	not copied	from inbound

So ConversationId no longer restarts at the interceptor hop and SagaId continues to flow on saga-spawned messages — both implicit gaps in the base bus's per-field copy that #2709's RouteBus constructor didn't address either.

The interceptor's DeliveryOptions payload is reduced to just GroupId, which is the only piece routing itself needs to read — every context field flows through the bus's overridden tracker.

Authorship

Lyall's original commit (a7788771f) is cherry-picked onto this branch unchanged so his test contribution (global_partitioning_tests.cs + MockWolverineRuntime router map) is preserved with his authorship. The implementation switch is a follow-up commit on top, co-authored with Lyall to credit the diagnosis.

The two ShouldNotContainKey calls in his tests were tweaked to disambiguate Shouldly's IDictionary / IReadOnlyDictionary overloads (Envelope.Headers is a concrete Dictionary that satisfies both) — behavioural assertions are otherwise verbatim.

Test plan

• GlobalPartitionedInterceptorTests: 16/16 on net9.0, including the four new behavioural tests from Preserve correlation/trace context across Global Partitioning interceptor #2709:
  • carries_inbound_correlation_id_onto_re_routed_envelope
  • carries_inbound_tenant_id_onto_re_routed_envelope
  • continues_inbound_distributed_trace_on_re_routed_envelope
  • does_not_copy_custom_inbound_headers_when_propagation_rule_not_configured
  • copies_only_allowlisted_custom_headers_when_propagation_rule_configured
• Full CoreTests.Runtime.Partitioning suite: 78/78 on net9.0 (no regressions in adjacent partitioning behaviour).

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