TST: enforce that assert_produces_warning checks the warning message

[jbrockmendel]

• closes TST: Disallow bare tm.assert_produces_warning #58290
• All code checks passed.

The enforcement half of GH-58290. The four content PRs it depended on — GH-67270, GH-67271, GH-67273, GH-67274 — have all merged, so this now runs clean; rebased onto their tips.

Adds a bare_assert_produces_warning check to scripts/validate_unwanted_patterns.py, wired up as a pre-commit hook scoped to pandas/tests/. It flags tm.assert_produces_warning(SomeWarning) with no match argument.

It also covers tm.maybe_produces_warning, which forwards **kwargs straight into assert_produces_warning. Without that, swapping one helper for the other is a one-word way around the check.

Exemptions. Calls asserting that nothing is raised — assert_produces_warning(None) and (False) — are exempt, since there is no message to match. Beyond that an explicit match=None opts out, which is roughly the tm.external_warning_produced escape hatch suggested in the issue; # pdlint: ignore[bare_warning_match] also works, following the existing pdlint convention in this script.

The only place needing an exemption today is the helper's own test file, and in each case the message assertion is unreachable by construction rather than merely inconvenient:

• TestFalseOrNoneExpectedWarning parametrizes false_or_none over [False, None], so no warning is expected at all — but the value is a name, not a literal, so a static check cannot see that.
• test_fail_to_catch_actual_warning and test_raises_during_exception never emit the expected class, so assert_produces_warning raises Did not see expected warning of class ... before it ever looks at match — which is exactly what those tests assert.

The other four calls in that file did have an assertable message and now check it.

Verified against the full tree. pre-commit run --all-files passes on the rebased branch with no files modified, so the check reports zero violations across every file in pandas/tests/ and lands with no exclusion list. (Cross-checked that it is not silently passing by feeding it a deliberately bare call, which it flags.)

scripts/tests/test_validate_unwanted_patterns.py covers both helper names, the tm.-qualified and bare forms, each exempt spelling, the implicit Warning default, and the pdlint comment.

[mroeschke]

Alternatively, could we change the signature of assert_produces_warning make expected_warning and match required arguments?

[jbrockmendel]

Good idea, will make a follow-up.