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/client/tool.rb

@@ -3,12 +3,13 @@
 module MCP
   class Client
     class Tool
-      attr_reader :name, :description, :input_schema
+      attr_reader :name, :description, :input_schema, :output_schema
 
-      def initialize(name:, description:, input_schema:)
+      def initialize(name:, description:, input_schema:, output_schema: nil)
         @name = name
         @description = description
         @input_schema = input_schema
+        @output_schema = output_schema
       end
     end
   end