fix: isolate TracesSampler callback failures

[elkampu]

Addresses the TracesSampler portion of #5535.

What changed

• isolate managed TracesSampler failures at the user-callback boundary and log TracesSampler callback failed. at error level
• return to the existing unconfigured-sampler fallback path, preserving inherited decisions before TracesSampleRate
• keep SDK-owned backpressure, random sampling, discard-reason, and DSC logic outside the recovery boundary
• apply the same callback isolation to the Android JNI and Cocoa native bridges, returning null on callback failure so the native SDK can use its normal fallback
• add managed regressions for static sample-in/sample-out, inherited sampled-in/sampled-out, exact error logging, and normal sample_rate client-report accounting

Validation

• dotnet test test/Sentry.Tests/Sentry.Tests.csproj -c Release -f net10.0 --filter "FullyQualifiedName~StartTransaction_TracesSampler" --no-restore — 10/10 passed
• dotnet build src/Sentry/Sentry.csproj -c Release -f net10.0 --no-restore — passed, 0 warnings/errors
• dotnet build src/Sentry/Sentry.csproj -c Release -f net10.0-android36.0 --no-restore — passed, 0 warnings/errors
• scoped dotnet format whitespace --folder --verify-no-changes for the five changed files — passed
• git diff --check — passed

Cocoa compilation requires macOS; the Cocoa change is limited to the same user-delegate recovery boundary and will be exercised by the repository's macOS CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.53%. Comparing base (98a6f8d) to head (2c705db).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5545      +/-   ##
==========================================
- Coverage   74.70%   74.53%   -0.17%     
==========================================
  Files         515      515              
  Lines       18948    18963      +15     
  Branches     3696     3694       -2     
==========================================
- Hits        14155    14134      -21     
- Misses       3909     3943      +34     
- Partials      884      886       +2     

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[jamescrosswell]

Thanks for the help @elkampu!

This looks 99% of the way there. I've just made one small suggestion.