feat!: Make the messaging provider handle scenario requests async

[adamrodger]

This removes a wrinkle in the API whereby scenario factory methods can be async internally, but because the messaging provider was sync we had to do a sync-over-async call to generate those scenarios.

Instead, now the messaging provider runs in an async context, which means the API has changed such that async scenario factory methods are now the default internally. Any sync scenario factories are wrapped such that they become async with Task.FromResult.

This creates a breaking API change because the verifier and the messaging provider were both previously sync disposable, whereas now they're async disposable so the the messaging prodiver can stop the async request handling context.

[Copilot]

Copilot reviewed 12 out of 17 changed files in this pull request and generated 1 comment.

Files not reviewed (5)

• samples/OrdersApi/Consumer.Tests/Consumer.Tests.csproj: Language not supported
• samples/OrdersApi/Provider.Tests/Provider.Tests.csproj: Language not supported
• src/PactNet.Abstractions/PactNet.Abstractions.csproj: Language not supported
• tests/PactNet.Abstractions.Tests/PactNet.Abstractions.Tests.csproj: Language not supported
• tests/PactNet.Tests/PactNet.Tests.csproj: Language not supported

Comments suppressed due to low confidence (1)

src/PactNet/Verifier/Messaging/MessagingProvider.cs:94

• Returning null from the Start method when the cancellation token is triggered could lead to a null reference error in the caller. Consider throwing an exception or restructuring the control flow to handle this case explicitly.

return null;