Add unit tests for Notes

Create notes_test.go with tests:
- TestNotes_List: list all notes on a task
- TestNotes_Create: create new note
- TestNotes_Update: update note comment
- TestNotes_Delete: delete note

Add testdata/notes/ fixtures:
- list.json: sample notes list

All 4 tests pass using httptest.Server mocking.

Closes checkvist-api-bbx

testdata/notes/list.json

@@ -0,0 +1,16 @@
+[
+  {
+    "id": 501,
+    "task_id": 101,
+    "comment": "First comment on task",
+    "updated_at": "2026-01-14T10:00:00Z",
+    "created_at": "2026-01-13T09:00:00Z"
+  },
+  {
+    "id": 502,
+    "task_id": 101,
+    "comment": "Second comment with more details",
+    "updated_at": "2026-01-14T11:30:00Z",
+    "created_at": "2026-01-14T11:00:00Z"
+  }
+]