Introduce ErrorResponse

Rather than passing a boolean flag to the response, introduce an
`ErrorResponse` class.

Author: pusewicz

lib/mcp.rb

@@ -19,6 +19,7 @@
 require_relative "mcp/tool"
 require_relative "mcp/tool/input_schema"
 require_relative "mcp/tool/response"
+require_relative "mcp/tool/error_response"
 require_relative "mcp/tool/annotations"
 require_relative "mcp/transport"
 require_relative "mcp/version"

lib/mcp/tool/error_response.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module MCP
+  class Tool
+    class ErrorResponse
+      def initialize(content) = super(content, error: true)
+    end
+  end
+end

test/mcp/tool/error_response.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class Tool
+    class ErrorResponseTest < ActiveSupport::TestCase
+      test "#initialize with content" do
+        content = [{
+          type: "text",
+          text: "Unauthorized",
+        }]
+        response = ErrorResponse.new(content)
+
+        assert_equal content, response.content
+        assert response.error?
+      end
+
+      test "#to_h" do
+        content = [{
+          type: "text",
+          text: "Unauthorized",
+        }]
+        response = ErrorResponse.new(content)
+        actual = response.to_h
+
+        assert_equal [:content, :isError].sort, actual.keys.sort
+        assert_equal content, actual[:content]
+        assert actual[:isError]
+      end
+
+      test "#error?" do
+        response = ErrorResponse.new(nil)
+        assert response.error?
+      end
+    end
+  end
+end