Add unit tests for Tasks

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

testdata/tasks/list.json

@@ -0,0 +1,34 @@
+[
+  {
+    "id": 101,
+    "checklist_id": 1,
+    "parent_id": 0,
+    "content": "First task",
+    "status": 0,
+    "position": 1,
+    "priority": 0,
+    "tags_as_text": "",
+    "due": "2026-01-20",
+    "assignee_ids": [],
+    "comments_count": 0,
+    "update_line": "",
+    "updated_at": "2026-01-14T10:00:00Z",
+    "created_at": "2026-01-10T09:00:00Z"
+  },
+  {
+    "id": 102,
+    "checklist_id": 1,
+    "parent_id": 0,
+    "content": "Second task",
+    "status": 1,
+    "position": 2,
+    "priority": 1,
+    "tags_as_text": "important, urgent",
+    "due": "",
+    "assignee_ids": [1, 2],
+    "comments_count": 3,
+    "update_line": "",
+    "updated_at": "2026-01-14T11:00:00Z",
+    "created_at": "2026-01-11T10:00:00Z"
+  }
+]