Fix audited messages retaining mutated headers

[w3lld1]

Summary

• snapshot incoming headers when the transport message is created
• restore the original header values before creating the audited message
• cover added, changed, and removed headers with unit and acceptance tests

Validation

• dotnet build src --configuration Release --no-restore
• all six test projects passed locally on Linux with .NET SDK 10.0.100 (NServiceBus.Core.Tests, NServiceBus.AcceptanceTests, NServiceBus.Core.Analyzer.Tests.Roslyn5, NServiceBus.TransportTests, NServiceBus.PersistenceTests, and NServiceBus.Learning.AcceptanceTests)

Fixes #7739

[CLAassistant]

All committers have signed the CLA.

[warwickschroeder]

Hi @w3lld1,

Thank you for your contribution! We’ve routed the PR to the appropriate team for evaluation.

If only small adjustments are needed, we’ll take care of them. If larger changes are required, we’ll either handle them ourselves (when we have capacity) or let you know what’s needed so you can choose whether to continue, or wait until we’re able to pick it up.

Kind regards,
The team at Particular

[mikeminutillo]

Hi @w3lld1,

We need to dedicate some time to investigate the impact of this PR as it will create a new dictionary for every single message that runs through the pipeline.

Can you tell us what prompted you to raise this PR? Are you encountering an issue with the current behavior of the audit feature?