Close outbox seam test-coverage gaps

[Vulthil]

Summary

• Add unit tests for TransactionalConsumeFilter (previously untested) and extend
  BrokerOutboxDispatcher/TransactionalPublishFilter coverage for happy publish/send
  paths, the missing-DestinationAddress error, and EnableTracing fidelity.
• Add OutboxProcessor tests for EnableParallelPublishing/MaxDegreeOfParallelism and
  failure isolation; every public OutboxProcessingOptions toggle now has at least one
  behavioral test (EnableMetrics and Retention.* were the remaining gaps).
• Add end-to-end integration tests on the existing TestHarness stack: a committed
  transaction is actually relayed and consumed with its stable message id preserved,
  and a rolled-back transaction is never relayed.

Verification

• Full solution build: clean, 0 warnings/errors, both TFMs.
• Vulthil.Messaging.Outbox.Tests: 16/16 passed (net9.0, net10.0).
• Vulthil.SharedKernel.Outbox.Tests: 36/36 passed (net9.0, net10.0).
• Vulthil.IntegrationTests (full suite, Docker-backed Postgres/MySql/Cosmos): 36/36
  passed (net9.0, net10.0); Docker was available and the new relay tests ran for real.
• No src/ or PublicAPI changes.

Backport to v1.0: no — test-only additions ship with the next release.