HTTP client does not handle problem+json error responses #1776

While reviewing the dataset migration, I found that HTTPClient does not handle the new server error format.

The server sends RFC 9457 problem responses as application/problem+json, with a string detail and an optional top-level code.

Currently:

• application/problem+json is sent to the XML parser.
• A JSON response with a string detail raises AttributeError, because the client expects detail to be a dictionary.

I reproduced this with a mocked missing-dataset response matching the server tests:

application/problem+json -> OpenMLServerError caused by ExpatError
application/json -> OpenMLServerError caused by AttributeError

This means the response code never reaches the existing exception mapping. For example, code 111 should reach OpenMLServerNoResult.

Could we recognize application/problem+json, read its top-level code and string detail, and keep the current XML and nested JSON formats working? I can take the small compatibility fix and add regression tests.

Related to #1491, which covers the broader exception work.