Soft confirm when creating práctica as operador with no asistente selected (companero field left empty)

Surfaced by support tickets #6/#7 (Ginette Morales Rangel, HC 86531): companeroId/asistente is optional at práctica creation with zero validation guard when rol=operador. A forgotten/empty selection saves silently as asistenteId=NULL with no warning, and (per PLUTO-544, still open) there's no way to edit-in the missing asistente afterward — so a forgotten selection is currently unrecoverable without a manual DB fix. Add a soft confirm step ('¿Confirmás que no hubo asistente/compañero en esta práctica?') when companero is empty at submit, so the omission is deliberate not accidental. Root-caused by coder-pluto-cc: no roster/timing bug, getClassmates is a plain unpaginated query, asistenteId=companeroId||null is faithful to what was submitted.