Add Tool output_schema support with comprehensive validation

This commit implements optional JSON Schema validation for tool outputs according to the MCP specification:

- Add MCP::Tool::OutputSchema class mirroring InputSchema functionality
- Support output_schema in class definitions, Tool.define, and OutputSchema objects
- Include validate_result method for runtime validation
- Update MCP::Client::Tool to track output schemas
- Add comprehensive test coverage with 249 passing tests
- Update README with dedicated Tool Output Schemas section
- Follow MCP spec requirements for server/client validation

lib/mcp/tool/output_schema.rb

@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+
+require "json-schema"
+
+module MCP
+  class Tool
+    class OutputSchema
+      class ValidationError < StandardError; end
+
+      attr_reader :properties, :required
+
+      def initialize(properties: {}, required: [])
+        @properties = properties
+        @required = required.map(&:to_sym)
+        validate_schema!
+      end
+
+      def ==(other)
+        other.is_a?(OutputSchema) && properties == other.properties && required == other.required
+      end
+
+      def to_h
+        { type: "object" }.tap do |hsh|
+          hsh[:properties] = properties if properties.any?
+          hsh[:required] = required if required.any?
+        end
+      end
+
+      def validate_result(result)
+        errors = JSON::Validator.fully_validate(to_h, result)
+        if errors.any?
+          raise ValidationError, "Invalid result: #{errors.join(", ")}"
+        end
+      end
+
+      private
+
+      def validate_schema!
+        check_for_refs!
+        schema = to_h
+        schema_reader = JSON::Schema::Reader.new(
+          accept_uri: false,
+          accept_file: ->(path) { path.to_s.start_with?(Gem.loaded_specs["json-schema"].full_gem_path) },
+        )
+        metaschema = JSON::Validator.validator_for_name("draft4").metaschema
+        errors = JSON::Validator.fully_validate(metaschema, schema, schema_reader: schema_reader)
+        if errors.any?
+          raise ArgumentError, "Invalid JSON Schema: #{errors.join(", ")}"
+        end
+      end
+
+      def check_for_refs!(obj = properties)
+        case obj
+        when Hash
+          if obj.key?("$ref") || obj.key?(:$ref)
+            raise ArgumentError, "Invalid JSON Schema: $ref is not allowed in tool output schemas"
+          end
+
+          obj.each_value { |value| check_for_refs!(value) }
+        when Array
+          obj.each { |item| check_for_refs!(item) }
+        end
+      end
+    end
+  end
+end