fix phantom missing fields for triggers schemas

[zzstoatzz]

on main, an example like this will complain about missing fields even though the schema has defaults the user doesn't need to pass

from prefect.events import DeploymentEventTrigger

if __name__ == "__main__":
    DeploymentEventTrigger(expect={"weeeeeee"})

it seems to be because pydantic needs default= in the Field definitions to recognize the default value as "not required" by the constructor of the instance

---

this might be a bug related to multiple-inheritance to report to pydantic but for now, this is a QoL improvement

[codspeed-hq]

CodSpeed Performance Report

Merging #15792 will not alter performance

_{Comparing fix-events-schemas-phantom-missing-fields (4dce3a9) with main (02b8219)}

Summary

✅ 3 untouched benchmarks

[desertaxle]

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