Update jfr-events module to follow style guide

[Copilot]

Updated the jfr-events module to comply with the OpenTelemetry Java Contrib style guide requirements with minimal changes:

Changes Made

1. Test method visibility: Removed public modifiers from test methods in JfrSpanProcessorTest to follow the style guide requirement that test methods should be package-protected rather than public.

2. Static import alignment: Updated the static import in the test class to use the recommended org.assertj.core.api.Assertions.assertThat instead of org.assertj.core.api.AssertionsForInterfaceTypes.assertThat as specified in the style guide.

Validation

• All existing tests continue to pass
• Code formatting verified with ./gradlew spotlessCheck
• Build verification completed successfully

The changes are minimal and surgical, addressing only the specific style guide compliance issues while preserving all existing functionality. The module's public classes were already properly declared as final and other style guide requirements were already met.

Fixes #363.

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