Update groups

Update up to BATCH_MAX_ITEMS groups in a single request.

Each item identifies a group via id (obfuscated, required). Updatable attributes: name, rule_email_domains, send_course_enrollment_email, category_id. At least one mutating field per item is required (enforced by the schema).

Within-batch duplicate detection is first-wins on id: the first occurrence of an id is processed; subsequent occurrences are pre-marked as duplicate_in_batch without reaching the service. This matches the POST endpoint's first-wins semantics on name.

A null value for category_id is meaningful — it clears the category assignment. Distinguishing "not provided" from "explicitly null" uses Pydantic's model_fields_set.

See api_v2/docs/batch-responses.md for the envelope shape.

Required OAuth scope: student-groups:write