[FIX] Show malformed XML response details

[anisha-04]

Metadata

• Reference Issue: Fixes Show invalid XML on ExpatError #986
• New Tests Added: Yes
• Documentation Updated: No
• Change Log Entry: "Show the response status and body when server error XML is malformed"

Details

What does this PR implement/fix?

When an unsuccessful GET response contains malformed XML, the HTTP client now preserves its existing retry behavior and ultimately raises OpenMLServerError with the request URL, HTTP status, and unparsed response body. The original XML parser exception remains attached as the exception cause.

A deterministic regression test mocks a malformed XML response and verifies the complete user-facing error.

Root cause

HTTPClient._validate_response() returned the raw ExpatError for malformed GET responses. After retries were exhausted, _request() raised that parser error directly, so the response details needed to diagnose a server-side failure were lost. Non-GET requests already produced a useful OpenMLServerError.

Testing

• .venv/bin/pytest -q tests/test_api/test_http.py — 13 passed
• .venv/bin/pytest -q tests/test_api/test_http.py -m 'not test_server' — 4 passed, 9 deselected
• .venv/bin/pre-commit run --files openml/_api/clients/http.py tests/test_api/test_http.py — all configured hooks passed (Ruff, Ruff format, mypy, and repository checks)
• git diff --check — passed

Documentation and compatibility

No documentation change is required because this does not alter the public API. Successful responses and valid server error responses are unchanged. Malformed GET error responses continue to be retried, but now fail with the existing OpenML exception type and useful diagnostic context.

Limitations

The raw response body is included only for malformed error responses, matching the existing non-GET behavior. This change does not alter retry policy or attempt to repair invalid server XML.

accept was accidental

[PGijsbers]

Thank you for your contribution! I simplified the flow of the method a bit and added additional checks for parsing "successful" responses since they are unfortunately not always valid XML.

Sorry for not taking the time to go through a review/mentoring cycle. Unfortunately I do not currently have the bandwidth to do that for many PRs in parallel, so I am prioritizing getting your changes in and getting the repository to a maintainable state. Thanks for your understanding!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 3.57143% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.68%. Comparing base (1349366) to head (6e52afd).

Files with missing lines	Patch %	Lines
openml/_api/resources/base/versions.py	4.54%	21 Missing ⚠️
openml/_api/clients/http.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1727      +/-   ##
==========================================
- Coverage   53.83%   53.68%   -0.16%     
==========================================
  Files          63       63              
  Lines        5240     5251      +11     
==========================================
- Hits         2821     2819       -2     
- Misses       2419     2432      +13     

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