fix(backport): protect UnsampledTransaction against double-Dispose

[Flash0ver]

Backports from v6 to v5:

• fix: protect UnsampledTransaction against double-Dispose #4722

#skip-changelog

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.51%. Comparing base (9895de7) to head (811eb78).
⚠️ Report is 2 commits behind head on support/v5.

Files with missing lines	Patch %	Lines
src/Sentry/Internal/UnsampledTransaction.cs	62.50%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           support/v5    #4854      +/-   ##
==============================================
+ Coverage       73.50%   73.51%   +0.01%     
==============================================
  Files             483      483              
  Lines           17692    17700       +8     
  Branches         3492     3493       +1     
==============================================
+ Hits            13005    13013       +8     
  Misses           3796     3796              
  Partials          891      891              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.