[Host.RabbitMq] Handle RabbitMQ RPC deserialization failures

[zarusz]

Summary

• send request/response error replies when request deserialization fails before handler invocation
• keep RabbitMQ from replaying background processing failures onto the next delivery, avoiding stuck unacked malformed messages
• add focused processor and concurrent decorator coverage

Fixes #480

Tests

• dotnet test src\Tests\SlimMessageBus.Host.Test\SlimMessageBus.Host.Test.csproj --no-restore -v minimal -p:UseSharedCompilation=false -m:1 -nodeReuse:false
• dotnet test src\Tests\SlimMessageBus.Host.RabbitMQ.Test\SlimMessageBus.Host.RabbitMQ.Test.csproj --no-restore -v minimal --filter "FullyQualifiedName!~IntegrationTests" -p:UseSharedCompilation=false -m:1 -nodeReuse:false

Note: full RabbitMQ integration tests require a local broker on 127.0.0.1:5672; without it they fail with BrokerUnreachableException.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
93.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud