Update a grant

Update an identity grant's metadata. Use this to change the associated policy, update the grantor, or adjust grant timestamps. **Updatable fields:** - `status` — transition the grant to a new status - `policy_id` or `policy_key` — reassign the grant to a different access policy - `granted_by` — update the grantor identifier - `granted_at` — correct the grant timestamp To revoke a grant, use the dedicated `POST /iga/grants/{grant_id}/revoke` endpoint instead — it handles deprovisioning workflows.