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Pull #2793

github

Add Grape::Mountable marker to identify a Grape app

Grape distinguished a mounted Grape app from a bare Rack app by duck-typing
on an incidental internal method (`respond_to?(:inheritable_setting)`).
That is brittle: Grape::API and Grape::API::Instance share no common
ancestor and don't even respond to the same methods (mount_instance vs
inheritable_setting/endpoints), so there was no single predicate and the
checks keyed on a settings method that has nothing to do with mountability.

Introduce a Grape::Mountable marker extended by both Grape::API and
Grape::API::Instance, giving one explicit `is_a?(Grape::Mountable)`
predicate. Route the identity checks through it: Endpoint#bare_rack_app?
(used for forward_match) and the mount branch in DSL::Routing.

Capability checks that go on to call a stage-specific method are left as
respond_to? on purpose (a Mountable does not respond to every such method):
`respond_to?(:mount_instance)` still gates the remountable-class stage,
`respond_to?(:endpoints)` still guards reading endpoints, and the settings
inheritance check still guards calling inheritable_setting.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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