Remove Grape::Util::ReverseStackableValues

[ericproulx]

Since #2795 internalized namespace_reverse_stackable into InheritableSetting, Grape::Util::ReverseStackableValues had a single consumer holding exactly two keys (rescue_handlers / base_only_rescue_handlers). Its only remaining job was reverse-order chain traversal — but InheritableSetting already tracks the same chain through #parent, so the class was a second way of walking a chain the setting has a pointer to.

This removes the class (and its spec) and replaces the store with a plain per-scope Hash of rescue_from registrations, merged recursively over #parent:

def merged_rescue_handlers(key)
  inherited = parent&.merged_rescue_handlers(key)
  own = @rescue_handler_maps&.[](key)
  return inherited unless own

  own.merge(inherited || {}) { |_klass, nearer, _inherited| nearer }
end

Hash#merge natively yields the ordering Middleware::Error's find-based scan depends on: the nearest scope's keys first (registration order preserved within a scope), inherited keys appended, nearer handler winning on collision. Behavior is unchanged, including the mounted-app specificity cases (prefers rescues defined by mounted even if outer is more specific) and same-scope ordering (rescue_from ArgumentError before rescue_from StandardError).

Side effects:

• InheritableSetting#to_hash now exposes rescue_handlers / base_only_rescue_handlers (the merged view callers actually consume) instead of a namespace_reverse_stackable key.
• Settings layers with no rescue_from no longer carry a ReverseStackableValues instance, and reads no longer build the intermediate arrays StackableValues#[] allocated per chain level.

🤖 Generated with Claude Code

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