test(taskService): Add comprehensive test cases for task service func…

…tions

- Added tests for createTask, getTasks, getTask, getTaskProblems, updateGradingStatus, updateTaskName, updateTaskDescription, and deleteTask functions.
- Ensured error handling scenarios are tested, including invalid task IDs, task not found, and task retrieval failures.
- Validated correct behavior for updating and retrieving tasks and problems.