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

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[zarusz]

@cord-agencyroot do you want to review this PR? It should fix one of the issues you've reported previously. Thanks.

[cord-agencyroot]

@zarusz looks good! thanks for fixing this