Output event name in SystemOutLogRecordExporter

[thswlsqls]

Fixes #8608

Description

• SystemOutLogRecordExporter.formatLog() did not print the log record's event name; it now appends [eventName: <name>] after the severity when one is set.
• This aligns the debug exporter with the OTLP log exporters, which already serialize LogRecord.EVENT_NAME (LogStatelessMarshaler, LogMarshaler).
• LogRecordData.getEventName() is a stable log data-model field (stabilized in Stabilize log record event name #7277); this exporter was not updated when the field was added.
• Output is unchanged when the event name is null or empty.

Testing done

• Added SystemOutLogRecordExporterTest#formatWithEventName for the event-name-present case; the existing format test covers the absent case.
• ./gradlew :exporters:logging:check — 16 tests passed.
• User-facing output change, so a ## Unreleased CHANGELOG entry is included. No public API change (formatLog is package-private), so no apidiff update.

Related: PR #6591 updated formatLog's body field when the data model changed.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.63%. Comparing base (6f6e146) to head (171d0bf).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...y/exporter/logging/SystemOutLogRecordExporter.java	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8609      +/-   ##
============================================
+ Coverage     91.62%   91.63%   +0.01%     
- Complexity    10321    10327       +6     
============================================
  Files          1013     1013              
  Lines         27287    27299      +12     
  Branches       3203     3206       +3     
============================================
+ Hits          25001    25015      +14     
+ Misses         1559     1558       -1     
+ Partials        727      726       -1     

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