Fail fast on three silent outbox misconfigurations

[Vulthil]

Summary

• A second outbox-enabled DbContext now throws at startup instead of silently orphaning the first context's messages (relay/retention only ever resolve one IOutboxStore).
• RelationalOutboxStore (Npgsql/MySql) now throws if the DbContext can't open a transaction, instead of silently relaying without one — previously FOR UPDATE SKIP LOCKED locks would release immediately, letting concurrent relay instances double-dispatch.
• Publishing inside an ambient System.Transactions.TransactionScope with no EF Core transaction now throws NotSupportedException instead of silently publishing directly while the scope is uncommitted (a ghost message on rollback). Documented as unsupported; full support is deferred.

Verification

• New/updated tests in Vulthil.SharedKernel.Outbox.Tests, a new Vulthil.SharedKernel.Infrastructure.Relational.Tests project, and Vulthil.Messaging.Outbox.Tests.
• Confirmed each failure mode reproduces on unfixed main and is fixed here.
• Full solution build (net9.0/net10.0) and all outbox-related test suites pass.
• WebApi sample and TestHost's single-registration path confirmed unaffected.

Backport to v1.0: no — behavior guards (silence becomes throwing) are minor-appropriate; ships in 1.1.0.