Share one KeyError path across rubies with and without detailed_message

[edgibbs]

Summary

👋 I ran into this while running bundle exec rake to get a baseline before
picking up an unrelated issue — the suite fails on a clean checkout of main.

main has been red since #1824 merged on 2026-08-21
(failing run).
Five jobs fail:

job	failure
Ruby 3.1 / Rails 7.0, 7.1, 7.2	12 specs
Ruby jruby-9.4 / Rails 7.0	12 specs
Run standard	3 Layout/* offenses

#1824 builds the trait-not-found message separately in each of two
Ruby-version branches, and the two drifted apart — one emits "trait". and
Referenced within, the other "trait" and referenced within. The specs
assert a single format, so every ruby taking the second branch fails.

The remaining failure is traits_spec.rb:497: error_options, added by #1824
so did_you_mean can suggest a factory's own traits, was only wired into the
Ruby >= 3.2 branch. On older rubies there was no suggestion data at all.

Why CI didn't catch it

#1824 came from a fork, so its workflow runs needed maintainer approval and
never got it — the PR shows no checks at all. The failure only surfaced on the
post-merge push to main.

The fix

did_you_mean writes its suggestions into KeyError#message before Ruby 3.2 and
into #detailed_message after. Rebuilding the error with the factory's own
traits added to the candidates regenerates them on older rubies and returns the
message untouched on newer ones — so one implementation serves both, and the
KeyError.method_defined?(:detailed_message) conditionals in Definition and
Registry are both deleted rather than repaired. Net −3 lines.

Dropping Registry's conditional also means every registry miss now carries
key and receiver on older rubies, where they previously raised
ArgumentError. Messages are unchanged; factory and strategy lookups simply
match the behaviour they already had on Ruby >= 3.2.

Testing

No new specs. The 12 added by #1824 already cover this — they fail on Ruby 3.1
and jruby-9.4 before this change and pass after. Adding more would duplicate
them.

Verified with bundle exec rake all_specs on:

• MRI 3.3.6 — 333 + 441 examples, 18 cucumber steps, standard clean
• MRI 3.1.7 — 774 examples
• JRuby 9.4.15.0 — 774 examples