add title field to prompt argument

`PromptArgument extends BaseMetadata`, which includes both name and
title
 - https://github.com/modelcontextprotocol/modelcontextprotocol/blob/51e093b8ebfde5e968879024566b7380d30c52d0/schema/2025-06-18/schema.ts#L686-L698
 - https://github.com/modelcontextprotocol/modelcontextprotocol/blob/51e093b8ebfde5e968879024566b7380d30c52d0/schema/2025-06-18/schema.ts#L294-l309

Author: topherbullock

README.md

@@ -624,6 +624,7 @@ class MyPrompt < MCP::Prompt
   arguments [
     MCP::Prompt::Argument.new(
       name: "message",
+      title: "Message Title",
       description: "Input message",
       required: true
     )
@@ -661,6 +662,7 @@ prompt = MCP::Prompt.define(
   arguments: [
     MCP::Prompt::Argument.new(
       name: "message",
+      title: "Message Title",
       description: "Input message",
       required: true
     )
@@ -692,6 +694,7 @@ server.define_prompt(
   arguments: [
     Prompt::Argument.new(
       name: "message",
+      title: "Message Title",
       description: "Input message",
       required: true
     )
@@ -718,7 +721,7 @@ e.g. around authentication state or user preferences.
 
 ### Key Components
 
-- `MCP::Prompt::Argument` - Defines input parameters for the prompt template
+- `MCP::Prompt::Argument` - Defines input parameters for the prompt template with name, title, description, and required flag
 - `MCP::Prompt::Message` - Represents a message in the conversation with a role and content
 - `MCP::Prompt::Result` - The output of a prompt template containing description and messages
 - `MCP::Content::Text` - Text content for messages

lib/mcp/prompt/argument.rb

@@ -3,17 +3,22 @@
 module MCP
   class Prompt
     class Argument
-      attr_reader :name, :description, :required, :arguments
+      attr_reader :name, :title, :description, :required
 
-      def initialize(name:, description: nil, required: false)
+      def initialize(name:, title: nil, description: nil, required: false)
         @name = name
+        @title = title
         @description = description
         @required = required
-        @arguments = arguments
       end
 
       def to_h
-        { name:, description:, required: }.compact
+        {
+          name: name,
+          title: title,
+          description: description,
+          required: required,
+        }.compact
       end
     end
   end

test/mcp/prompt_test.rb

@@ -113,7 +113,12 @@ def template(args, server_context:)
         title: "Mock Prompt",
         description: "a mock prompt for testing",
         arguments: [
-          Prompt::Argument.new(name: "test_argument", description: "Test argument", required: true),
+          Prompt::Argument.new(
+            name: "test_argument",
+            title: "Test argument title",
+            description: "This is a test argument description",
+            required: true,
+          ),
         ],
       ) do |args, server_context:|
         content = Content::Text.new(args["test_argument"] + " user: #{server_context[:user_id]}")