Create tasks_test.go with comprehensive tests:
- TestTasks_List: list all tasks in checklist
- TestTasks_Get: get single task by ID
- TestTasks_Create: create basic task
- TestTasks_Create_WithBuilder: create task with all options
- TestTasks_Update: update task properties
- TestTasks_Delete: delete task
- TestTasks_Close: mark task as completed
- TestTasks_Reopen: reopen closed task
- TestTasks_Invalidate: invalidate task
- TestDueDate_Parsing: table-driven due date parsing tests
- TestTaskBuilder: builder pattern validation
Add testdata/tasks/ fixtures:
- list.json: sample task list
- single.json: single task response
All 11 tests pass using httptest.Server mocking.
Closes checkvist-api-v2f
