Remove students from a group

JSON:API envelope for DELETE /v2/groups/{id}/relationships/students/.

Mirrors :class:MembershipAddRequestEnvelope: data is always an array (single-remove is a list of one), and cap enforcement (<= DEFAULT_BATCH_MAX_ITEMS items) lives in the endpoint via _validate_batch_envelope — 400 via ValidationError, not Pydantic 422.

207 Multi-Status response envelope for batch-delete operations.

Distinct from BatchResultEnvelope so the generic T on the latter stays bound to a real resource type and consumers reading BatchSucceededItem.result keep their non-null contract.

Items appear in the same order as the request inputs. summary.succeeded counts BatchDeletedItem rows — the field name reflects "operation did not fail" rather than the verb-specific outcome (which is on every item's own status discriminator).