fix: get_benchmark_results rejects a filter on a column the chosen kind lacks

[propcgamer20-png]

Problem

_get_benchmark_results_impl (faircode/mcp_server.py) silently no-ops a filter whose column isn't in the selected kind's CSV. results_performance.csv has no protected_attribute column, so:

a = _get_benchmark_results_impl(kind="performance")
b = _get_benchmark_results_impl(kind="performance", protected_attribute="gender")
a["total_matches"], b["total_matches"], a == b
# -> 315 315 True

The caller asked for gender-specific performance numbers and got the whole unfiltered table back, with no way to tell from the response that the filter was dropped. A bogus value on a real column correctly returns total_matches: 0, so the tool already distinguishes "no match" from "filter applied" - just not "filter targets a column this kind doesn't have".

Fix

Raise ValueError when a non-None filter names a column absent from the selected kind's CSV - the same contract profile_dataset / compare_datasets use for an overrides column that isn't in the dataset:

'protected_attribute' is not a column of the 'performance' benchmark results
(available: audit, strategy, model, metric, value, ci_low, ci_high, n);
it cannot be used as a filter for this kind

audit, strategy, model, metric exist in both frozen CSVs, so the only reachable case is protected_attribute vs kind="performance". Docstring updated.

Tests

test_get_benchmark_results_performance_kind_ignores_protected_attribute_filter (which asserted the silent-ignore behavior) is replaced by test_get_benchmark_results_rejects_filter_on_column_absent_from_kind, which checks the raise for performance and that the same filter still works against fairness.

pytest tests/test_mcp_server.py -> 57 passed, 1 failed. The failure (test_get_explainer_returns_content_and_metadata) is pre-existing on main on this platform (a \ vs / path check in _get_explainer_impl) and unrelated to this change. ruff clean.

Closes #512

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@yakew7 @ahmdkaml - new PR touching a path you own, please review.