fix(instrumentation-logging): promote otel.event.name to LogRecord.event_name in _translate

[rajdhruvsingh]

Description

LoggingHandler._translate never set the first-class event_name field on
LogRecord. When a user passed otel.event.name (or the deprecated event.name)
via extra={...}, it silently landed in attributes as a plain key and was
exported as a regular OTLP attribute instead of the dedicated event_name field.

This fix updates _get_attributes to pop both keys from the attributes dict and
return the resolved event name alongside the attributes. _translate then passes
it as event_name= to LogRecord(...).

• otel.event.name (stable) takes precedence
• event.name (deprecated) is used only as a fallback
• Both keys are always popped so neither leaks into attributes

Fixes #4743

Part of open-telemetry/community#1751

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Five new unit tests added to TestLoggingHandler in tests/test_handler.py:

• test_otel_event_name_promoted_to_event_name_field — stable key is promoted to event_name, removed from attributes
• test_deprecated_event_name_promoted_as_fallback — deprecated event.name works when stable key is absent
• test_otel_event_name_takes_precedence_over_deprecated_event_name — stable key wins when both are present, both popped from attributes
• test_event_name_is_none_when_not_provided — event_name is None when neither key is passed (no regression)
• test_other_extra_attributes_unaffected_by_event_name_promotion — unrelated extra fields still land in attributes normally

All 36 existing tests continue to pass (41 total).

To reproduce:
python3 -m pytest instrumentation/opentelemetry-instrumentation-logging/tests/test_handler.py -v

Does This PR Require a Core Repo Change?

• No.

Checklist

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated