Fix sign extension on LogRecord flags in low-allocation log marshaler

[thswlsqls]

Fixes #

Description

• LogStatelessMarshaler passes the TraceFlags byte directly into the int-taking serializeFixed32 / sizeFixed32 for LogRecord.flags, so a flags byte with the high bit set sign-extends.
• 0x80 is then written little-endian as 80 FF FF FF instead of 80 00 00 00, and the JSON renders the sign-extended integer. fixed32 is always 4 bytes, so the size is unaffected — only the byte/JSON content is wrong.
• Fix: use serializeByteAsFixed32 / sizeByteAsFixed32, which mask ((int) value) & 0xff, matching the non-stateless sibling LogMarshaler (lines 121, 156).

Testing done

• Set the shared SPAN_CONTEXT in LowAllocationLogRequestMarshalerTest to TraceFlags.fromByte((byte) 0x80), so the existing relative assertions (validateOutput, validateJsonOutput) compare the stateless and non-stateless paths with the high bit set. Both fail before the fix and pass after.
• ./gradlew :exporters:otlp:common:check passed (118 tests).
• Internal package, so no docs/apidiffs/ change. Added a ## Unreleased CHANGELOG.md entry under SDK > Exporters.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (a7a2f98).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8493   +/-   ##
=========================================
  Coverage     78.77%   78.77%           
  Complexity     8579     8579           
=========================================
  Files          1009     1009           
  Lines         28993    28993           
  Branches       3599     3599           
=========================================
  Hits          22839    22839           
  Misses         5311     5311           
  Partials        843      843           

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