Fix .NET worker cancellation example

[brianstrauch]

Summary

• pass an infinite-delay task to the .NET worker quick-start example
• use the Func<Task> overload of TemporalWorker.ExecuteAsync
• keep the worker running indefinitely without requiring a cancellation token

Validation

• compiled the complete example with .NET SDK 8.0.424 and Temporalio 1.18.0
• git diff --check

Closes #238