fix(mcp): proxy_hints says in notes why an empty hints list is empty (#1044)

[minutechreview]

Summary

The MCP proxy_hints tool returned {"hints": []} both when every pair was tested and none was significant, and when the dataset had fewer than two dimensions so no pair could be tested. #1022 fixed this in the terminal and HTML reports; the MCP result still did not say which it was.

When hints is empty, _proxy_hints_impl now adds one entry to notes, using _no_hints_text from faircode/report.py so the words match the reports:

• none tested: a pair needs at least two dimensions (found N) when fewer than two dimensions exist
• none found: no pair of dimensions is significantly associated otherwise

A column given through held_out_with is compared with every dimension, so it counts toward the two a pair needs. A result with at least one hint has no such note. faircode/SPEC.md section 11 describes it.

Tests in tests/test_mcp_server.py cover one dimension, two independent dimensions, and a result with a hint (no note).

Type

• Bug fix

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Linked issue

Closes #1044

Checks: pytest tests -m "not slow" --ignore=tests/browser gives 907 passed, 19 skipped. ruff check faircode tests scripts and scripts/check_em_dash.py pass. The new one-dimension test fails if the note is removed. I did not run the slow benchmark tests or the browser tests.

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