检查用户可分配性。Check user assignability.

检查指定用户是否有权限被分配为该 Issue 的处理人。若该用户可被分配，返回 204 无内容响应；若不具分配资格（如非仓库协作者），返回 404 错误。可在分配处理人之前进行权限预校验。 Check whether a specific user can be assigned as an assignee of the issue. Returns 204 No Content if the user can be assigned; returns 404 if the user is not eligible (e.g., not a repository collaborator). Can be used to pre-validate permissions before assigning. 访问令牌调用此接口需包含以下权限。Required permissions for access token. repo-issue:r