[OpenTelemetry] Edge cases for parsing OTEL_TRACES_SAMPLER_ARG

[Kielek]

Fixes Codex analysis finding

Changes

• Fixed OTEL_TRACES_SAMPLER_ARG handling to treat out-of-range, NaN, and
  infinite values as invalid and fall back to the default ratio when using
  traceidratio and parentbased_traceidratio samplers.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• [ ] Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.79%. Comparing base (77665f0) to head (55fb17d).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7103      +/-   ##
==========================================
- Coverage   88.83%   88.79%   -0.05%     
==========================================
  Files         270      270              
  Lines       12929    12933       +4     
==========================================
- Hits        11486    11484       -2     
- Misses       1443     1449       +6     

Flag	Coverage Δ
unittests-Project-Experimental	88.39% <100.00%> (-0.37%)	⬇️
unittests-Project-Stable	88.77% <100.00%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry/Trace/TracerProviderSdk.cs	99.35% <100.00%> (+<0.01%)	⬆️

... and 3 files with indirect coverage changes