fix: Fix incorrect variable assignment for 'sampled' key

[xjshi]

In SentryTraceContext, the 'sampled' key was incorrectly using _sampleRate.

📜 Description

In SentryTraceContext.m, the 'sampled' key was incorrectly using _sampleRate. This resulted in malformed trace contexts being sent to Sentry, potentially breaking sampling decisions in distributed tracing.

[itaybre]

@xjshi thanks for the fix!

Would you mind adding a test for this? Or would like me to push the required tests for this? (to this doesn't happen again)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.738%. Comparing base (23936a7) to head (0c53284).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #7120       +/-   ##
=============================================
+ Coverage   84.691%   84.738%   +0.047%     
=============================================
  Files          459       459               
  Lines        27494     27494               
  Branches     12121     12126        +5     
=============================================
+ Hits         23285     23298       +13     
+ Misses        4169      3937      -232     
- Partials        40       259      +219     

Files with missing lines	Coverage Δ
Sources/Sentry/SentryTraceContext.m	96.078% <100.000%> (ø)

... and 32 files with indirect coverage changes

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[xjshi]

@itaybre Test added. It verifies all fields in the dictionary returned by the serialize method to ensure correct mapping.

[philprime]

@sentry review