Add missing fields to mock SNEWS events

We got a Sentry error where a field was missing: https://ligo-caltech.sentry.io/issues/6239128442/?environment=test&project=1425216&query=is%3Aunresolved%20issue.priority%3A%5Bhigh%2C%20medium%5D&referrer=issue-stream&statsPeriod=7d&stream_index=18&utc=true

This seems due to the difference of the expected SNEWS event vs our constructed mock SNEWS events:

Real:

    "extra_attributes": {
        "GRB": {
            "author_ivorn": "ivo://nasa.gsfc.tan/gcn",
            "dec": 0.0,
            "designation": null,
            "redshift": null,
            "how_description": "SNEWS: SuperNova Early Warning System",
            "coord_system": "UTC-FK5-GEO",
            "trigger_id": "1000500",
            "error_radius": 360.0,
            "how_reference_url": "http://gcn.gsfc.nasa.gov/snews.html",
            "ra": 0.0,
            "ivorn": "ivo://nasa.gsfc.gcn/SNEWS#Event2025-01-21T17:00:01.00_1000500-837",
            "trigger_duration": null,
            "author_shortname": "SNEWS (via VO-TAN)",
            "T90": null,
            "observatory_location_id": "GEOLUN"
        }
    }

Mock:

"extra_attributes": {}

The solution is add some of these missing attributes when constructing mock SNEWS events in tasks.first2years_external.py, perhaps by giving a VOEvent template that is closer to a real one with these missing fields.