add _meta field to Prompt

> See [General fields: `_meta`](/specification/2025-06-18/basic/index#meta) for notes on `_meta` usage.

51e093b8eb/schema/2025-06-18/schema.ts (L680-L683)

lib/mcp/prompt.rb

@@ -8,13 +8,20 @@ module MCP
       attr_reader :title_value
       attr_reader :description_value
       attr_reader :arguments_value
+      attr_reader :meta_value
 
       def template(args, server_context: nil)
         raise NotImplementedError, "Subclasses must implement template"
       end
 
       def to_h
-        { name: name_value, title: title_value, description: description_value, arguments: arguments_value.map(&:to_h) }.compact
+        {
+          name: name_value,
+          title: title_value,
+          description: description_value,
+          arguments: arguments_value&.map(&:to_h),
+          _meta: meta_value,
+        }.compact
       end
 
       def inherited(subclass)
@@ -23,6 +30,7 @@ module MCP
         subclass.instance_variable_set(:@title_value, nil)
         subclass.instance_variable_set(:@description_value, nil)
         subclass.instance_variable_set(:@arguments_value, nil)
+        subclass.instance_variable_set(:@meta_value, nil)
       end
 
       def prompt_name(value = NOT_SET)
@@ -61,7 +69,15 @@ module MCP
         end
       end
 
-      def define(name: nil, title: nil, description: nil, arguments: [], &block)
+      def meta(value = NOT_SET)
+        if value == NOT_SET
+          @meta_value
+        else
+          @meta_value = value
+        end
+      end
+
+      def define(name: nil, title: nil, description: nil, arguments: [], meta: nil, &block)
         Class.new(self) do
           prompt_name name
           title title
@@ -70,6 +86,7 @@ module MCP
           define_singleton_method(:template) do |args, server_context: nil|
             instance_exec(args, server_context:, &block)
           end
+          meta meta
         end
       end