Calendar PATCH schema rejects null needed to clear date/dateTime for in-place conversion #951

gws calendar events patch rejects JSON null during schema validation, so a PATCH can't clear start.date/end.date when converting an all-day event to a timed one (or dateTime going the other way).

Repro (gws 0.22.5, no credentials needed):

gws calendar events patch \
  --params '{"calendarId":"synthetic-test","eventId":"synthetic-event"}' \
  --json '{"start":{"date":null,"dateTime":"2026-10-10T10:00:00-07:00"},"end":{"date":null,"dateTime":"2026-10-10T11:00:00-07:00"}}' \
  --dry-run

error[validation]: Request body failed schema validation:
- end.date: Expected type 'string', found null
- start.date: Expected type 'string', found null

Without the null, the existing date is kept and the Calendar API rejects the request with Invalid start time.

Expected: null accepted for optional fields in PATCH bodies, since that is how the API clears them.

Workaround: events get, replace start/end, then events update (full PUT).