Fix(Entity): Improve block arity detection for Symbol#to_proc

Addresses `ArgumentError`s when using `&:method_name` with `expose`,
a scenario particularly affected by Ruby 3.0's stricter argument handling
for procs. The previous arity check, including the condition
`block.parameters == [[:req], [:rest]]`, was not consistently reliable
for these cases.

This change introduces `determine_block_arity` which:
1. Attempts to parse the original method name from `Proc#to_s` for
   blocks likely created via `&:method_name`.
2. If the method name is found and the object responds to it, this
   logic uses the *actual arity of the original method* to correctly
   determine how `instance_exec` should be called.
3. If the original method name can't be determined, it falls back
   to using `block.parameters.size`.

This ensures methods exposed via `&:method_name` are called with the
correct arguments (i.e., with or without `options`), resolving the
`ArgumentError`s and removing the need for the previous `rescue` logic.

Author: numbata

lib/grape_entity/entity.rb

@@ -518,21 +518,26 @@ def serializable_hash(runtime_options = {})
       root_exposure.serializable_value(self, opts)
     end
 
+    def determine_block_arity(block)
+      # As from Ruby 3.0, the block parameters are always equal to `[[:req], [:rest]]`
+      # In that case the method name could be extracted from the block to find the original method arity.
+      origin_method_name = block.to_s.scan(/(?<=\(&:)[^)]+(?=\))/).first&.to_sym
+
+      if origin_method_name && object.respond_to?(origin_method_name, true)
+        object.method(origin_method_name).parameters.size
+      else
+        block.parameters.size
+      end
+    end
+
     def exec_with_object(options, &block)
-      if block.parameters.count == 1 || block.parameters == [[:req], [:rest]]
+      block_arity = determine_block_arity(block)
+
+      if block_arity == 0
         instance_exec(object, &block)
       else
         instance_exec(object, options, &block)
       end
-    rescue StandardError => e
-      # it handles: https://github.com/ruby/ruby/blob/v3_0_0_preview1/NEWS.md#language-changes point 3, Proc
-      # accounting for expose :foo, &:bar
-      if e.is_a?(ArgumentError) && block.parameters == [[:req], [:rest]]
-        Rails.logger.error("***** ERROR in exec_with_object: #{e.message}\n#{e.backtrace.join("\n")}")
-        raise Grape::Entity::Deprecated.new e.message, 'in ruby 3.0'
-      end
-
-      raise e
     end
 
     def exec_with_attribute(attribute, &block)

spec/grape_entity/entity_spec.rb

@@ -423,28 +423,16 @@ def raises_argument_error
           end
 
           context 'with block passed in via &' do
-            if RUBY_VERSION.start_with?('3')
-              specify do
-                subject.expose :that_method_without_args, &:method_without_args
-                subject.expose :method_without_args, as: :that_method_without_args_again
-
-                object = SomeObject.new
-                expect do
-                  subject.represent(object).value_for(:that_method_without_args)
-                end.to raise_error Grape::Entity::Deprecated
-
-                value2 = subject.represent(object).value_for(:that_method_without_args_again)
-                expect(value2).to eq('result')
-              end
-            else
-              specify do
-                subject.expose :that_method_without_args_again, &:method_without_args
+            specify do
+              subject.expose :that_method_without_args, &:method_without_args
+              subject.expose :method_without_args, as: :that_method_without_args_again
 
-                object = SomeObject.new
+              object = SomeObject.new
+              value1 = subject.represent(object).value_for(:that_method_without_args)
+              expect(value1).to eq('result')
 
-                value2 = subject.represent(object).value_for(:that_method_without_args_again)
-                expect(value2).to eq('result')
-              end
+              value2 = subject.represent(object).value_for(:that_method_without_args_again)
+              expect(value2).to eq('result')
             end
           end
         end