Pin minitest below 6

[JuanVqz]

Summary

Pin minitest < 6. minitest 6 is incompatible with Rails 8.0.2's test runner and breaks the suite on boot.

Root cause

minitest 6 restructured its runner. The per-method runner is now Minitest::Runnable.run(klass, method_name, reporter) (3 args) — in minitest 5 that was run_one_method, while run(reporter, options) was the suite-level runner (2 args).

Rails 8.0.2's Rails::LineFiltering#run(reporter, options = {}) override was written against the old 2-arg shape. Under minitest 6, run_suite calls run(self, method_name, reporter), hitting that 1..2-arity override and raising:

ArgumentError: wrong number of arguments (given 3, expected 1..2)
  railties-8.0.2/lib/rails/test_unit/line_filtering.rb:7:in 'run'

before any test runs.

Why pin instead of fix forward

The fix lives in railties' line_filtering.rb, not our app. Adopting minitest 6 requires a Rails release that supports it (8.1+), which is beyond our rails ~> 8.0.2 pin and belongs in a dedicated Rails upgrade. A standalone app-side monkeypatch would mean re-implementing line filtering against minitest 6 internals — fragile and not worth it.

Impact

Without this pin, Dependabot re-resolutions keep pulling minitest 6.0.6 and red-flagging the test job across every batch of dependency PRs (4 open PRs are currently red for exactly this reason). Pinning makes the "upgrade to latest compatible" strategy safe.

Follow-up

Revisit minitest 6 as part of a future Rails 8.1 upgrade, then drop this pin.

Verification

• bundle lock keeps minitest at 5.25.5.
• Full test suite: 33 runs, 101 assertions, 0 failures, 0 errors.