Fix task return and apply correct disposal pattern for FileSystemMainDomLock

[JasonElkin]

Prerequisites

• I have added steps to test this contribution in the description below

Firstly:

This PR adds a fix to a bug I introduced in #18119 😬

Using an async method inside Task.Factory.StartNew() results in the outer task being returned, instead of the inner long-running task. When the outer task completes (which it does pretty much right away) the MainDom releases the lock early.

Adding an Unwrap() returns the inner task so that the _listenCompleteTask inside MainDom fires only when the inner task is finished.

Secondly:

This PR adds a similar Dispose pattern as used by the SqlMainDomLock in order to call the cancellation token and stop the long-running task when Disposed by the MainDom at shutdown.

Testing

Drop a breakpoint inside _listenCompleteTask .
Before the changes in this PR, that breakpoint will be hit erroneously soon after boot.
After this PR it will be hit only when the lock is intentionally released, or at shutdown.

[github-actions]

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[AndyButland]

This looks good to me, thanks @JasonElkin.

I've verified with the test steps. Had trouble getting my breakpoints to be hit but having added a log line into _listenCompleteTask I can see the error of this being called just after boot with the code in contrib but not once this PR is applied.