[Splay Crons] Let cron schedules omit the minute in the SDK (ENG-10691) (#54721)

Structured cron schedules (`hourly`, `daily`, `weekly`, and `monthly`) can now omit `minuteUTC`. When omitted, Convex chooses a stable start time within the scheduled hour, which avoids concentrating jobs at the top of the hour. The `crons.hourly` helper can also omit its schedule object entirely.

The dashboard renders these schedules using the actual next run time and reconstructs schedule literals without an `undefined` minute.

This depends on #54596 for the backend scheduling behavior.

GitOrigin-RevId: e1d793468484ebf2fa39a2fb855eb641ab9f8e0b