Fix delayed message null activity

[jpalac]

Addresses #7209

[jpalac]

There is a sample provided to test this scenario: https://github.com/jonesr-out/nservicebus-otel-bug

It seems that when the StartActivity runs with default values, there is no active listener so a null activity is returned from this call.

This PR makes a change to pass in the traceFlags of the spanContext into the new root activity context. It seems to solve the issue reported in the sample.

I have created a test to try and reproduce the issue, however I was unable to get the StartActivity code to return null the way it does in the demo.
I believe that may be due to our testing setup of the activity listener.
I'm not sure if there's another way that this could be setup to be tested.

@lailabougria @SzymonPobiega can you see anything wrong with the proposed solution here? Any possible side effects?