fix: remove default marker assignment in EventTag

[tcheeric]

Summary

• stop EventTag from pre-setting marker so it remains null unless explicitly provided
• add tests verifying default marker omission and explicit marker serialization/deserialization
• document event tag deserialization without marker

Testing

• mvn -q verify (fails: Could not find a valid Docker environment; see logs)

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https://chatgpt.com/codex/tasks/task_b_68a7329a2a7083318066373b0b8dab5d

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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