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.

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