Batch add and update facility specialties

Applies a batch of specialty updates and additions to a facility in a single request. Use this when you need to modify multiple specialty associations at once; for a single new specialty, use POST /facilities/{id}/specialties instead. Updates modify existing TenantFacilitySpecialty records (specialty FK, effective date, isPrimary) and are applied as one all-or-nothing batch — if that batch call fails, the entire request fails, with no partial-success path for updates. Additions create new specialty associations at the facility level and are processed independently — a failure on one addition is captured in the response's errors array and does not stop or fail the other additions.