rest-api: `Transfer-Encoding: chunked` fails with 500

[slint]

When the Transfer-Encoding HTTP header is set to chunked, the request fails with an internal server error:

Here’s an example of a failing request:

curl -X POST https://sandbox.zenodo.org/api/deposit/depositions \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \ 
  -H "Transfer-Encoding: chunked" \
  -d '{"metadata": {"title": "Test", "doi": null, "publication_date": "2025-09-05", "description": "", "access_right": "restricted", "creators": [{"name": "John Smith"}], "license": "mit", "upload_type": "dataset"}}'

The same request minus the Transfer-Encoding header succeeds.