test: cover issue todo, issue link, weight and iteration events

• Adds tests for IssuesService.CreateTodo and IssueLinksService.GetIssueLink.
• Adds pagination-query and error-path tests for the issue weight and iteration events. No production code changes.

Each test checks the request method and path, the decoded response, an invalid ID type, a failing request option and a 404. The list tests also check that page and per_page are sent.