[OpenTelemetry] Logs - Return None for unset or out-of-range LogLevel mapping

[ysolomchenko]

Fixes # N/A
Design discussion issue # N/A

Found by Codex/security scans.

Changes

Fixed LogRecord.LogLevel to preserve LogLevel.None and handle unspecified or out-of-range severities without returning invalid enum values.

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 88.73%. Comparing base (7f78716) to head (b6dc613).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7092      +/-   ##
==========================================
+ Coverage   88.69%   88.73%   +0.03%     
==========================================
  Files         270      270              
  Lines       12919    12920       +1     
==========================================
+ Hits        11458    11464       +6     
+ Misses       1461     1456       -5     

Flag	Coverage Δ
unittests-Project-Experimental	88.37% <100.00%> (-0.31%)	⬇️
unittests-Project-Stable	88.72% <100.00%> (+0.07%)	⬆️

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

Files with missing lines	Coverage Δ
src/OpenTelemetry/Logs/LogRecord.cs	70.86% <100.00%> (+0.19%)	⬆️

... and 4 files with indirect coverage changes

[github-actions]

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