unsupported /files endpoint returns a 404 instead of a 501

A user tried to upload a file with the Python client but this feature is not supported by the back-end and he got a 404 back:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/openeo/rest/connection.py", line 1488, in upload_file
    resp = self.put(f"/files/{target!s}", expected_status=200, data=f)
  File "/usr/local/lib/python3.9/site-packages/openeo/rest/connection.py", line 259, in put
    return self.request("put", path=path, data=data, headers=headers, allow_redirects=False, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/openeo/rest/connection.py", line 769, in request
    return _request()
  File "/usr/local/lib/python3.9/site-packages/openeo/rest/connection.py", line 762, in _request
    return super(Connection, self).request(
  File "/usr/local/lib/python3.9/site-packages/openeo/rest/connection.py", line 168, in request
    self._raise_api_error(resp)
  File "/usr/local/lib/python3.9/site-packages/openeo/rest/connection.py", line 188, in _raise_api_error
    raise OpenEoApiError(
openeo.rest.OpenEoApiError: [404] NotFound: 404 Not Found: The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again. (ref: r-2402229ec6924eceb2e402f908d9cd18)

This should have returned a 501 with code FeatureUnsupported instead.

Locate the /files endpoint handling in the driver and compare it with other unsupported-feature responses. Reproduce the Python client's upload request, then verify that an unsupported endpoint returns HTTP 501 with the FeatureUnsupported code instead of 404.