Use autoload to defer loading of unused subsystems

## Motivation and Context

`lib/mcp.rb` currently declares all `require_relative` statements in one place.
As a result, Server and Client are both loaded, the stdio and Streamable HTTP
transports are both loaded, and Tool, Prompt, and Resource are all loaded,
regardless of which components are actually used.

These components are selected by the user depending on their use case,
so loading everything upfront is unnecessary.

Using `autoload` for selected constants defers loading until the constant
is actually referenced, so only the subsystems required by the user are loaded.

## How Has This Been Tested?

All existing tests pass unchanged.

## Breaking Change

None. This change only defers loading of unused components.

lib/mcp.rb

@@ -1,38 +1,22 @@
 # frozen_string_literal: true
 
 require_relative "json_rpc_handler"
-require_relative "mcp/annotations"
 require_relative "mcp/configuration"
-require_relative "mcp/content"
-require_relative "mcp/icon"
-require_relative "mcp/instrumentation"
-require_relative "mcp/methods"
-require_relative "mcp/progress"
-require_relative "mcp/prompt"
-require_relative "mcp/prompt/argument"
-require_relative "mcp/prompt/message"
-require_relative "mcp/prompt/result"
-require_relative "mcp/resource"
-require_relative "mcp/resource/contents"
-require_relative "mcp/resource/embedded"
-require_relative "mcp/resource_template"
-require_relative "mcp/server"
-require_relative "mcp/server_context"
-require_relative "mcp/server/transports/streamable_http_transport"
-require_relative "mcp/server/transports/stdio_transport"
 require_relative "mcp/string_utils"
-require_relative "mcp/tool"
-require_relative "mcp/tool/input_schema"
-require_relative "mcp/tool/output_schema"
-require_relative "mcp/tool/response"
-require_relative "mcp/tool/annotations"
 require_relative "mcp/transport"
 require_relative "mcp/version"
-require_relative "mcp/client"
-require_relative "mcp/client/http"
-require_relative "mcp/client/tool"
 
 module MCP
+  autoload :Annotations, "mcp/annotations"
+  autoload :Client, "mcp/client"
+  autoload :Content, "mcp/content"
+  autoload :Icon, "mcp/icon"
+  autoload :Prompt, "mcp/prompt"
+  autoload :Resource, "mcp/resource"
+  autoload :ResourceTemplate, "mcp/resource_template"
+  autoload :Server, "mcp/server"
+  autoload :Tool, "mcp/tool"
+
   class << self
     def configure
       yield(configuration)

lib/mcp/client.rb

@@ -1,5 +1,8 @@
 # frozen_string_literal: true
 
+require_relative "client/http"
+require_relative "client/tool"
+
 module MCP
   class Client
     # Initializes a new MCP::Client instance.

lib/mcp/prompt.rb

@@ -1,5 +1,9 @@
 # frozen_string_literal: true
 
+require_relative "prompt/argument"
+require_relative "prompt/message"
+require_relative "prompt/result"
+
 module MCP
   class Prompt
     class << self

lib/mcp/resource.rb

@@ -1,5 +1,8 @@
 # frozen_string_literal: true
 
+require_relative "resource/contents"
+require_relative "resource/embedded"
+
 module MCP
   class Resource
     attr_reader :uri, :name, :title, :description, :icons, :mime_type

lib/mcp/server.rb

@@ -4,6 +4,9 @@ require_relative "../json_rpc_handler"
 require_relative "instrumentation"
 require_relative "methods"
 require_relative "logging_message_notification"
+require_relative "progress"
+require_relative "server_context"
+require_relative "server/transports"
 
 module MCP
   class ToolNotUnique < StandardError

lib/mcp/server/transports.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+module MCP
+  class Server
+    module Transports
+      autoload :StdioTransport, "mcp/server/transports/stdio_transport"
+      autoload :StreamableHTTPTransport, "mcp/server/transports/streamable_http_transport"
+    end
+  end
+end

lib/mcp/server/transports/stdio_transport.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
-require_relative "../../transport"
 require "json"
+require_relative "../../transport"
 
 module MCP
   class Server

lib/mcp/server/transports/streamable_http_transport.rb

@@ -1,8 +1,8 @@
 # frozen_string_literal: true
 
-require_relative "../../transport"
 require "json"
 require "securerandom"
+require_relative "../../transport"
 
 module MCP
   class Server

lib/mcp/tool.rb

@@ -1,5 +1,10 @@
 # frozen_string_literal: true
 
+require_relative "tool/annotations"
+require_relative "tool/input_schema"
+require_relative "tool/output_schema"
+require_relative "tool/response"
+
 module MCP
   class Tool
     class << self