Execute a Dynamics 365 Dataverse OData batch request with changesets for atomic multi-table writes

Steps

1. Construct a multipart/mixed request body with a batch boundary string; the Content-Type header is multipart/mixed; boundary=batch_<unique-id>
2. Wrap related write operations (POST, PATCH, DELETE) that must succeed or fail together inside a changeset part: Content-Type: multipart/mixed; boundary=changeset_<unique-id>
3. Each individual request inside the changeset is a Content-Type: application/http part with its own HTTP verb, relative URL, and JSON body
4. POST the entire batch to /api/data/vX.X/$batch; Dataverse processes the changeset atomically — if any operation in the changeset fails, all are rolled back
5. Parse the multipart response: each part maps to the corresponding request, with its own HTTP status code and body — iterate parts in order to correlate responses to requests
6. To reference a record created earlier in the same changeset by a Content-ID, use $<contentId> in the URL of a later request within the same changeset (e.g., /$1/parentaccountid@odata.bind)