[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)