[FEATURE] Write unit tests for Activity Event features

[umtsndg]

📌 Description

Ensure every created event strictly follows Activity Streams 2.0 specifications through backend tests.

✅ Acceptance Criteria

• All events include @context, type, actor, object, and published.
• Invalid or incomplete events are rejected.
• Automated test suite runs successfully.
• Validation errors are descriptive.

📋 Tasks

• Write unit tests that create various AS2 events.
• Test for missing or malformed fields.
• Add AS2 schema validation helper.
• Integrate validation tests into CI/CD pipeline.

👥 Assignees

• @umtsndg will complete the tasks.
• @akcineren will review.