Fall back to observed timestamp in SystemOutLogRecordExporter

[thswlsqls]

Fixes #8942

Description

• SystemOutLogRecordExporter.formatLog() printed 1970-01-01T00:00:00Z for any log record emitted without setTimestamp(...), because it only read getTimestampEpochNanos(), which is 0 in that case.
• It now uses the observed timestamp when Timestamp is 0, following the logs data model guidance for single-timestamp formats: "Use Timestamp if it is present, otherwise use ObservedTimestamp." EventToSpanEventBridge (sdk-extensions/incubator) uses the same fallback.
• No field is added and the format is unchanged. Records that set a timestamp (including log4j/logback/JUL appender bridges) print as before.
• Spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/logs/data-model.md#field-observedtimestamp

Testing done

• Added SystemOutLogRecordExporterTest#formatWithoutTimestampUsesObservedTimestamp (timestamp unset, observed timestamp set). With the fix reverted it fails: expected 1970-08-07T10:00:00Z ... but was 1970-01-01T00:00:00Z ....
• Existing format() and formatWithEventName() set a timestamp and leave the observed timestamp at 0; they still pass, so Timestamp keeps priority.
• ./gradlew :exporters:logging:check passed (17 tests). No apidiff change: formatLog is package-private.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.66%. Comparing base (387f5c5) to head (b4664c9).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8944      +/-   ##
============================================
+ Coverage     90.65%   90.66%   +0.01%     
- Complexity    10887    10890       +3     
============================================
  Files          1015     1015              
  Lines         29731    29734       +3     
  Branches       3876     3877       +1     
============================================
+ Hits          26953    26959       +6     
  Misses         1860     1860              
+ Partials        918      915       -3     

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