获取 Issue。Get issue.

根据 Issue 编号获取指定仓库中的单个 Issue 详情，包括标题、描述、状态、标签、处理人、优先级等完整信息。如果 Issue 不存在或当前用户无权访问，将返回 404 错误。 Get the full details of a single issue in the specified repository by issue number, including title, description, state, labels, assignees, priority, etc. Returns 404 if the issue does not exist or the user lacks access. 访问令牌调用此接口需包含以下权限。Required permissions for access token. repo-issue:r