Fix ReadWriteLogRecord default getObservedTimestampEpochNanos returning record timestamp

[thswlsqls]

Fixes #8501

Description

• ReadWriteLogRecord.getObservedTimestampEpochNanos() default returned toLogRecordData().getTimestampEpochNanos() instead of getObservedTimestampEpochNanos(), so implementations relying on the interface defaults got the record timestamp instead of the observed timestamp.
• Matches the SDK implementation SdkReadWriteLogRecord.getObservedTimestampEpochNanos(), which returns the observedTimestampEpochNanos field, and the sibling default getTimestampEpochNanos(), which is correct. LogRecordData exposes both getters separately.
• The slip was introduced when the default getters were added in Add getters/accessors for readable fields in ReadWriteLogRecord. #6924.

Testing done

• Added ReadWriteLogRecordTest#defaultGetObservedTimestampEpochNanos_returnsObservedTimestamp, which exercises the interface defaults via a custom ReadWriteLogRecord whose LogRecordData has distinct timestamp (100) and observed timestamp (200). Fails before the fix (was 100), passes after.
• ./gradlew :sdk:logs:check passed.
• Added a ## Unreleased entry in CHANGELOG.md (replace #XXXX with the PR number).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (aa6aec0).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8504   +/-   ##
=========================================
  Coverage     78.77%   78.77%           
- Complexity     8579     8580    +1     
=========================================
  Files          1009     1009           
  Lines         28993    28993           
  Branches       3599     3599           
=========================================
  Hits          22839    22839           
+ Misses         5311     5310    -1     
- Partials        843      844    +1     

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