Clean up ensure_block_arity! dead returns and fix splat-arg edge case

Use block.arity == 1 instead of block.parameters.one? so that
splat-arg lambdas (->(*args) { }) receive both object and options
instead of being misrouted to the single-arg path.

Remove dead return values from ensure_block_arity! since the caller
no longer uses its return value — it is now purely a validation method.

Author: numbata

lib/grape_entity/entity.rb

@@ -528,7 +528,7 @@ def exec_with_object(options, &block)
       if symbol_to_proc_wrapper?(block)
         ensure_block_arity!(block)
         instance_exec(object, &block)
-      elsif block.parameters.one?
+      elsif block.arity == 1
         instance_exec(object, &block)
       else
         instance_exec(object, options, &block)
@@ -539,7 +539,7 @@ def ensure_block_arity!(block)
       # MRI currently always includes "( &:foo )" for symbol-to-proc wrappers.
       # If this format changes in a new Ruby version, this logic must be updated.
       origin_method_name = block.to_s.scan(/(?<=\(&:)[^)]+(?=\))/).first&.to_sym
-      return 0 unless origin_method_name
+      return unless origin_method_name
 
       unless object.respond_to?(origin_method_name, true)
         raise ArgumentError, <<~MSG
@@ -548,7 +548,7 @@ def ensure_block_arity!(block)
       end
 
       arity = object.method(origin_method_name).arity
-      return 0 if arity.zero?
+      return if arity.zero?
 
       raise ArgumentError, <<~MSG
         Cannot use `&:#{origin_method_name}` because that method expects #{arity} argument#{'s' if arity != 1}.

spec/grape_entity/entity_spec.rb

@@ -500,6 +500,18 @@ def raises_argument_error
               expect(value).to eq('result')
             end
           end
+
+          context 'with splat-argument lambda' do
+            it 'passes the object and options' do
+              subject.expose :args_count do |*args|
+                args.size
+              end
+
+              object = SomeObject.new
+              value = subject.represent(object).value_for(:args_count)
+              expect(value).to eq(2)
+            end
+          end
         end
 
         context 'with no parameters passed to the block' do