Fix outbox failed batch retry loop

[zarusz]

Summary

• Fixes [Host.RabbitMq] Outbox retries exhaust quickly when broker is down #459 by stopping failed outbox batches from immediately re-entering the same sending pass.
• Failed messages still have delivery attempts recorded, but the next retry is governed by the outbox polling/lock timing instead of a tight same-instance loop.
• Adds core outbox unit coverage and PostgreSQL/Testcontainers integration coverage for the retry behavior.

Tests

• dotnet test src\Tests\SlimMessageBus.Host.Outbox.Test\SlimMessageBus.Host.Outbox.Test.csproj --no-build -v minimal -m:1
• dotnet test src\Tests\SlimMessageBus.Host.Outbox.PostgreSql.Test\SlimMessageBus.Host.Outbox.PostgreSql.Test.csproj --no-build -v minimal -m:1