Fix REST API validation failures for omitted fields

[rasmi]

Commit f27f7fc introduced strict TypeBox Value.Check validation for REST API endpoints. This caused Python client payloads (which omit fields rather than explicitly sending null due to Pydantic's exclude_none=True) to be rejected with Expected union value 400 errors for any fields defined strictly as Type.Union([Type.Null(), ...]).

This commit fixes the mismatch by wrapping all nullable TypeBox schemas in Type.Optional(), allowing clients to perform partial updates or omit empty fields cleanly while maintaining strict runtime typing.

[jimbojw]

I created #1118 to track the upstream problem. Is there an easy way to test for this at PR time? I believe we already run Firebase emulators for testing the functions. We should be able to also exercise the Python CLI at this time.