fix: Convert Swift extension to helper class for use by ObjC

[maxchuquimia]

📜 Description

Resolves a crash in #3931

💡 Motivation and Context

Changes requested by @philipphofmann here: #3931 (comment)

💚 How did you test it?

• Existing unit tests pass

📝 Checklist

You have to check all boxes before merging:

• I reviewed the submitted code.
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.868%. Comparing base (3912b16) to head (b3a6377).

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3973       +/-   ##
=============================================
+ Coverage   90.824%   90.868%   +0.044%     
=============================================
  Files          594       594               
  Lines        46078     46082        +4     
  Branches     16444     16456       +12     
=============================================
+ Hits         41850     41874       +24     
+ Misses        4158      4027      -131     
- Partials        70       181      +111     

Files	Coverage Δ
Sources/Sentry/SentryNetworkTracker.m	96.393% <100.000%> (+0.023%)	⬆️
Sources/Swift/Tools/URLSessionTaskHelper.swift	100.000% <100.000%> (ø)
Tests/SentryTests/URLSessionTaskHelperTests.swift	100.000% <100.000%> (ø)

... and 37 files with indirect coverage changes

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[philipphofmann]

I fixed the Changelog and removed the public keyword for the helper class to merge this faster. Thanks a lot, @maxchuquimia 💯. LGTM.