Add tests for submitting hierarchy of nodes

A hierarchy of nodes can be submitted to an existing root node with PUT `/nodes/{node_id}` endpoint request.
Implement unit tests and e2e tests for the endpoint.

Locate the existing unit and end-to-end test structure for the PUT `/nodes/{node_id}` endpoint. First inspect how requests submit a hierarchy to an existing root node, then define tests covering that behavior and run the relevant test suites. Done means both unit and e2e coverage verifies successful hierarchy submission.

索引モデルが issue の本文から書いたものです。