Add spec for issue: polymorphic entity with custom documentation

- Reproduces "Empty model" error when using Grape::Entity with all hidden properties
- Tests custom documentation override with Array[Object] type
- Includes working example showing expected behavior
- Follows existing issue spec patterns in the codebase

Author: numbata

.rspec

@@ -1,2 +1,3 @@
 --format documentation
 --color
+--require "spec_helper"

spec/issues/962_polymorphic_entity_with_custom_documentation_spec.rb

@@ -0,0 +1,122 @@
+# frozen_string_literal: true
+
+describe '#962 empty entity with custom documentation type' do
+  context "when entity has no properties" do
+    let(:app) do
+      Class.new(Grape::API) do
+        namespace :issue962 do
+          class Foo < Grape::Entity
+          end
+
+          class Report < Grape::Entity
+            expose :foo,
+              as: :bar,
+              using: Foo,
+              documentation: {
+                type: 'Array[object]',
+                desc: 'The bar in your report',
+                example: {
+                  'id' => 'string',
+                  'status' => 'string',
+                }
+              }
+          end
+
+          desc 'Get a report', success: Report
+          get '/' do
+            present({ foo: [] }, with: Report)
+          end
+        end
+
+        add_swagger_documentation format: :json
+      end
+    end
+
+    subject(:swagger_doc) do
+      get '/swagger_doc'
+      JSON.parse(last_response.body)
+    end
+
+    specify do
+      expect(swagger_doc['definitions']['Report']['properties']['bar']).to eql({
+        'type' => 'array',
+        'description' => 'The bar in your report',
+        'items' => {
+          '$ref' => '#/definitions/Foo'
+        },
+        'example' => {
+          'id' => 'string',
+          'status' => 'string'
+        }
+      })
+    end
+
+    specify do
+      expect(swagger_doc['definitions']['Foo']).to eql({
+        'type' => 'object',
+        'properties' => {},
+      })
+    end
+  end
+
+  context "when entity has only hidden properties" do
+    let(:app) do
+      Class.new(Grape::API) do
+        namespace :issue962 do
+          class Foo < Grape::Entity
+            expose :required_prop, documentation: { hidden: true }
+            expose :optional_prop, documentation: { hidden: true }, if: ->() { true }
+          end
+
+          class Report < Grape::Entity
+            expose :foo,
+              as: :bar,
+              using: Foo,
+              documentation: {
+                type: 'Array[object]',
+                desc: 'The bar in your report',
+                example: {
+                  'id' => 'string',
+                  'status' => 'string',
+                }
+              }
+          end
+
+          desc 'Get a report', success: Report
+          get '/' do
+            present({ foo: [] }, with: Report)
+          end
+        end
+
+        add_swagger_documentation format: :json
+      end
+    end
+
+    subject(:swagger_doc) do
+      get '/swagger_doc'
+      JSON.parse(last_response.body)
+    end
+
+    specify do
+      expect(swagger_doc['definitions']['Report']['properties']['bar']).to eql({
+        'type' => 'array',
+        'description' => 'The bar in your report',
+        'items' => {
+          '$ref' => '#/definitions/Foo'
+        },
+        'example' => {
+          'id' => 'string',
+          'status' => 'string'
+        }
+      })
+    end
+
+    it "hides optional properties only" do
+      expect(swagger_doc['definitions']['Foo']).to eql({
+        'type' => 'object',
+        'properties' => {},
+        'required' => ['required_prop'],
+      })
+    end
+  end
+end