fix: protect UnsampledTransaction against double-Dispose

[Flash0ver]

Fixes #4719

• Finish Method Executes Redundantly #4719

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Follow-up to #4717.
Merge into version6 branch after merging #4717.

Mimicking implementation of Sentry.TransactionTracer.

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#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@             Coverage Diff             @@
##             version6    #4722   +/-   ##
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  Coverage            ?   73.26%           
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  Files               ?      480           
  Lines               ?    17433           
  Branches            ?     3440           
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  Hits                ?    12772           
  Misses              ?     3815           
  Partials            ?      846           

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[alexsohn1126]

None of the tests that are added seems to look for the double-dispose use case that this PR is trying to prevent.

Maybe something like DoubleDispose_DoseNothing?

[Flash0ver]

None of the tests that are added seems to look for the double-dispose use case that this PR is trying to prevent.

Maybe something like DoubleDispose_DoseNothing?

The test UnsampledTransactionTests.Dispose_Finished_DoesNothing does verify that: when Disposing (or Finishing) a Finished (or Disposed) UnsampledTransaction again, a Hub-member was only invoked once.
I wanted to keep both the naming as well as the behavior of the test consistent with TransactionTracer/TransactionTracerTests.