Translate ValidationErrors#full_messages once instead of on every call

[ericproulx]

Summary

ValidationErrors builds its #message from #full_messages when it is created. #full_messages then translated every error again on each call: an I18n lookup for the grape.errors.format string (with interpolation) and one per attribute name. Each is a few microseconds on I18n 1.15.

The README's recipe for answering with the list, below, therefore paid for every translation twice:

rescue_from Grape::Exceptions::ValidationErrors do |e|
  error!({ messages: e.full_messages }, 400)
end

The list is now kept from the first call and handed out as a copy, so a caller changing it cannot change what the next caller gets. #message is still built through #full_messages, so a subclass overriding it keeps working as before.

Benchmarks

Median of 7 interleaved subprocess rounds, Ruby 4.0.6. The API uses the README handler above, with requires :id, type: Integer and requires :name, type: String.

request	no JIT	YJIT
one param missing	+21.9%	+24.9%
both params missing	+27.3%	+34.4%
default handler, missing param	−0.1%
default handler, invalid Integer	−0.1%

Behaviour

The list now stays what #message was built from. Before, a handler that switched the locale and then asked for the list got a mix of both locales. With a French format and a French presence message, I18n.with_locale(:fr) { e.full_messages } answered ["id : is missing", "nick : is missing"] for an error raised in English: the French format and untranslated attribute names around the English messages, which were translated when each Validation was raised. It now answers ["id is missing", "Nickname is missing"], the same text as #message. I did not add an UPGRADING entry, since the old output was half-translated anyway; happy to add one.

Otherwise responses are byte-identical to master over:

• a 120-request matrix of rescue_from ValidationErrors handlers reading full_messages (once, twice, and after changing the list), message, both together, as_json, to_json, errors, error!(e), and re-raising. It includes translated attribute names, a request in another locale, mutually_exclusive (several params in one error) and values. The locale-switching handler above is the only difference.
• the 228-case error-path matrix and the 104-case and 108-case validation matrices from earlier PRs.

Missing spec, added

A caller changing the array #full_messages returned must not change the next answer. The spec passes on master, where each call builds a new array, and fails if the copy is dropped.

Test plan

• Full RSpec suite passes locally.
• RuboCop clean.
• Mutation-checked: dropping the copy fails the new spec. Dropping the memo is invisible by design, and the benchmark covers it.
• CI green.

🤖 Generated with Claude Code

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Markdowns

* [#XXXX](https://github.com/ruby-grape/grape/pull/XXXX): Translate `ValidationErrors#full_messages` once instead of on every call - [@ericproulx](https://github.com/ericproulx).
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[ericproulx]

Closing in favour of #2936, which combines #2922–#2934 into one PR, re-benchmarked as a whole against master. The write-up here (behaviour matrix, mutation results) still describes this part of the change.