Correct specialty on a tenant practitioner specialty record

Replaces the specialty identifier on a tenant practitioner specialty record. This is a data-correction operation — it does not trigger termination events, lifecycle changes, or downstream notifications. Use it when the wrong specialty was recorded and needs correcting without invoking the termination lifecycle; do not use it to end a practitioner's association with a specialty, use PUT /practitioners/tenant-practitioner-specialties/terminate for that. The specialty record must already exist, and the replacement specialty must be a valid, active tenant specialty.