Add unit tests for Checklists

Create checklists_test.go with comprehensive tests:
- TestChecklists_List: list all checklists
- TestChecklists_ListArchived: list with archived filter
- TestChecklists_Get: get single checklist by ID
- TestChecklists_Get_NotFound: handle 404 error
- TestChecklists_Create: create new checklist
- TestChecklists_Update: update checklist name
- TestChecklists_Delete: delete checklist

Add testdata/checklists/ fixtures:
- list.json: sample checklist list
- list_archived.json: archived checklists
- single.json: single checklist response

All 7 tests pass using httptest.Server mocking.

Closes checkvist-api-347

testdata/checklists/list.json

@@ -0,0 +1,24 @@
+[
+  {
+    "id": 1,
+    "name": "My First Checklist",
+    "public": false,
+    "archived": false,
+    "read_only": false,
+    "task_count": 10,
+    "task_completed": 3,
+    "tags_as_text": "",
+    "updated_at": "2026-01-14T10:00:00Z"
+  },
+  {
+    "id": 2,
+    "name": "Work Tasks",
+    "public": true,
+    "archived": false,
+    "read_only": false,
+    "task_count": 25,
+    "task_completed": 12,
+    "tags_as_text": "work, important",
+    "updated_at": "2026-01-13T15:30:00Z"
+  }
+]