Split the grant write ops off the generalized v2 write ops

V2WriteOps composes V2GrantWriteOps, so a caller holding the generalized write ops can hand out and take back entity grants. Grants carry permission caps, which belongs beside the permission ops (BA-7560) rather than in the ops every entity write path holds.

Scope:
- Remove V2GrantWriteOps from the V2WriteOps composition and hand it out through its own provider context.
- Move the 6 call sites over: vfolder/repository.py (5), entity_invitation/repository.py (1), plus the repository test.
- V2WriteOps keeps entity, global, field, dangling field, update and batch writes.

JIRA Issue: BA-7580

Start by locating the V2WriteOps and V2GrantWriteOps composition and provider contexts. Update the five call sites in vfolder/repository.py, the call site in entity_invitation/repository.py, and the repository test. Done means grant operations come from their own provider context while V2WriteOps retains the listed entity, global, field, dangling-field, update, and batch writes.

Written by the indexing model from the issue text.