fix: beforeSend overriding default values even if they were not set

[buenaflor]

📜 Description

We need to keep the KMP event model in sync with the native one. Users mainly edit events in KMP hooks like beforeSend, so we must track whether each field was actually modified:
If a field was changed, propagate the new value to the native event.• If it was untouched, leave the native default in place.
Previously we overwrote native defaults with null values by default e.g., Cocoa’s auto‑detected release was not working since we set a null release

💡 Motivation and Context

Fixes #337

💚 How did you test it?

TBD unit tests

📝 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.

🔮 Next steps

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.32%. Comparing base (ff8969f) to head (de816fd).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #376      +/-   ##
==========================================
- Coverage   79.92%   79.32%   -0.61%     
==========================================
  Files          44       11      -33     
  Lines         812      324     -488     
  Branches      108       49      -59     
==========================================
- Hits          649      257     -392     
+ Misses        107       39      -68     
+ Partials       56       28      -28     

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[buenaflor]

I'll update this PR, let's fix it only for the release field now. I'm not even sure if the other fields need to be fixed like that as well

[buenaflor]

can only test cocoa like this since the SentryEvent in java is final and we don't use a mocking framework yet