/practicas global-landing student picker: lightweight id/nombre/apellido-only query

PLUTO-502 follow-up (flagged by coder at impl). On the GLOBAL/admin no-filter /practicas landing, the new student search picker is powered by getStudentsForTeacher(null) which runs the full StudentSummary roster query on the default view - heavier than needed just to populate a name->id picker. Works fine on current data. If that landing's latency becomes a concern, add a lightweight picker-only read returning just id/nombre/apellido (not the full StudentSummary shape). Not blocking; conditional perf optimization.