Fix: VariantCollectionCoercer#to_s returns "Array[Type, ...]" notation

[bogdan]

Problem

type: [Integer, String] and types: [Integer, String] have different semantics:

• types: [Integer, String] — the parameter itself can be Integer or String (variant types)
• type: [Integer, String] / type: Array[Integer, String] — the parameter is always an Array whose members can be Integer or String (variant-member-type collection)

However, VariantCollectionCoercer had no custom to_s, so route.params[:type] for the type: form serialised to a garbage object string ("#<Grape::Validations::Types::VariantCollectionCoercer:0x...>"). Documentation tools such as grape-swagger and grape-oas could not distinguish it from the plain "[Integer, String]" produced by types:, and could not generate a correct schema for either case.

Fix

Add VariantCollectionCoercer#to_s returning the Grape DSL notation:

• type: [Integer, String] → "Array[Integer, String]"
• type: Set[Integer, String] → "Set[Integer, String]"
• types: [Integer, String] → "[Integer, String]" (unchanged)

This matches the notation users write in the DSL and gives documentation tools an unambiguous signal to emit {type: array, items: {oneOf: [...]}} rather than a plain oneOf.

Compatibility

• declared_params_handler.rb reads route.params[:type] but only matches 'Array', 'Hash', 'Set', and the single-type [Type] pattern — "Array[Integer, String]" does not collide with any of those branches.
• grape-swagger's type-parsing regexes (/\[(.*)\]$/, /\A\[.*\]\z/) are unaffected.

[github-actions]

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[ericproulx]

Nice fix. Two asks: (1) add a spec for the Set form (type: Set[Integer, String] → "Set[Integer, String]") — it's an untested branch. (2) The compatibility note isn't quite right for Set: "Set[Integer, String]" does match declared_params_handler.rb:111's start_with?('Set'), so a type: Set[…] param now declares Set.new instead of falling through. That looks like the correct/consistent behaviour, but please call it out and add a declared-params test to pin it down.