Prácticas: sometimes a student is operador with NO student asistente (single-participant práctica) - define canonical rule

Surfaced by the Matías Wang case: Dayeli (operador) confirmed no other student participated - i.e. a práctica can exist with an operador but no asistente. Elazar flags this as a distinct data/workflow problem: is an asistente required? What is the canonical participant rule for a real clinical práctica (operador-only allowed vs operador+asistente mandatory)? Affects create-time validation + how single-participant prácticas are counted in stats/distribution (PLUTO-507). NEEDS A BIGGER TALK WITH CLIENT to set the rule before any enforcement. Related: PLUTO-78, PLUTO-507. Blocked on client discussion.