fix: prevent hanging if stop called from multiple threads

[robertcoltheart]

Description

There are certain cases when StopAsync is called multiple times on different threads, which presents itself as the StopAsync call hanging indefinitely. This change adds thread protection to ensure that StopAsync is only called once.

An example of when this occurs is here when using WebApplicationFactory<> for testing.

How Has This Been Tested?

I've run this in a local unit test scenario against a real Kafka instance to ensure that the teardown is clean and handles multiple concurrent calls to StopAsync.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have made corresponding changes to the documentation

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