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Look up an entity by the object's own class before its element's (#2844)

Grape::DSL::Entity#object_class decided which class to look an entity up for
by duck-typing:

    return object.klass if object.respond_to?(:klass)
    return object.first.class if object.respond_to?(:first)
    object.class

Both tests are answered by plenty of single objects, and the object's own
class was consulted only last. A Struct is Enumerable, so it responds to
#first; so does any model that includes Enumerable; and #klass is hardly
exclusive to ActiveRecord::Relation. For all of those, `represent Model,
with: Entity` was silently ignored and the entity for whatever #first
returned was looked up instead -- usually nothing, so the raw object was
serialized.

Try the object's own class first and fall back to the collection or wrapped
class only when that comes up empty. An Array still resolves through its
element class, since Array itself has no entity, and a relation still
resolves through #klass.

Deferring the fallback also stops #first from being called at all when the
object resolves on its own class, which matters for anything where taking
the first element is expensive or has side effects.

Longstanding: the duck-typing dates to 2014 (v0.10.0) and behaves the same
way in 3.3.4.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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