Fix the test suite against unreleased Grape 4.0

[ericproulx]

Summary

Two Grape 4.0 changes break the suite against grape=HEAD. Both are intentional upstream and documented there, so the fixes belong here.

Before this PR, GRAPE_VERSION=HEAD bundle exec rspec fails 23 examples; after, the suite is green on both grape=HEAD (4.0.0) and grape 3.3.4.

1. Collection custom types need parse (12 failures)

ruby-grape/grape#2817 builds an Array/Set element coercer eagerly, in the coercer's constructor, so that coercers — shared across requests — create no state at request time. A collection whose element type Grape cannot coerce is therefore rejected while the params block is evaluated, instead of on the first request that supplies the parameter:

ArgumentError: type Entities::ApiError should support coercion via `[]`

Entities::ApiError is an OpenStruct / Representable::Decorator with no parse, so type: Array[Entities::ApiError] now raises as the API class loads. Fixed by giving the fixtures a one-argument parse, which is what the bare type: X fixtures already carry.

Why this was invisible until now. params_array_spec only ever GETs /swagger_doc/... — it never POSTs to the route declaring that type. Grape used to build the element coercer lazily, so nothing ever built it. Had a request supplied the parameter, grape 3.3.4 answered 400 {"error":"array_of_entities is invalid"} — the ArgumentError raised inside the coercer and swallowed by the coercion validator. So this was a latent misdeclaration in the fixtures, not new breakage.

Comment correction. The existing fixture comments credited this to "Grape 3.2+". That's right for bare type: X but wrong for the collection form:

grape	type: X	Array[X]
3.1.0	OK	OK
3.2.0	ArgumentError	OK
3.3.4	ArgumentError	OK
4.0	ArgumentError	ArgumentError

Comments now point at Grape 4.0 / #2817, and UPGRADING's custom-type bullet — which currently only covers the bare form — gains the collection case.

2. StackableValues is a read-only view (11 failures)

ruby-grape/grape#2823 made Grape::Util::StackableValues a read-only view: .new now takes (new_values, inherited_values) and []= is gone. swagger_routing_spec built instances directly and then wrote to them, so it fails on both counts.

combine_namespaces only ever reads keys off the object ([](key) returning an Array, for .last and .join('/')), so these specs now stub that narrow contract rather than constructing a real instance. That's version-agnostic and can't break again on either the constructor or the writer — the same reasoning behind the stackable_values_double helper already in request_param_parsers/route_spec.rb, which covers the other 9.

Note the fixture values changed shape slightly: the old []= pushed, so [:mount_path] = ['//foo/', '/bar'] actually produced [['//foo/', '/bar']]. The doubles use the flat form, which is what real Grape produces (add_mount_path pushes one string per mount level). Assertions are unchanged — Array#join recurses, so both shapes yield the same joined path.

Grape's compatibility surface is unaffected: grape-swagger's lib never constructs a StackableValues, it only reads through one (is_a?, #inherited_values, #new_values, #[]), all of which #2823 preserved deliberately.

Verification

• Green on grape=HEAD (4.0.0) and grape 3.3.4.
• Example count unchanged from master (530, verified with --dry-run against both trees) — no coverage dropped.
• I could not run the grape 2.1 row locally (grape 2.1 doesn't install on Ruby 4.0.5); leaving that to CI.
• rubocop could not run: .rubocop.yml references Style/OneClassPerFile, which the installed RuboCop rejects. Pre-existing — it fails the same way on untouched files like lib/grape-swagger.rb.

🤖 Generated with Claude Code

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[ericproulx]

@numbata you need to merge this PR first to fix grape=HEAD tests.

[numbata]

@dblock I will merge this and do release.