Add "Draft" protocol version to supported protocol versions

For example, the `title` introduced in #109 is not supported in the latest stable protocol version "2025-06-18":
https://modelcontextprotocol.io/specification/2025-06-18

It is supported in the Draft of the next version:
https://modelcontextprotocol.io/specification/draft

In other words, the fact that the Ruby SDK treats "2025-06-18" as the default protocol version while `title` is
only supported in the Draft results in an inconsistency.

Because of this, a new version `"Draft"` has been added to the list of supported versions.
Its value follows the schema below and is defined as `DRAFT-2025-v3`:
14ec41c/schema/draft/schema.ts (L15)

It was also considered to continue using the latest stable version "2025-06-18" as the Ruby SDK default instead of the Draft.
However, in that case, users would need to explicitly specify `MCP::Server.new(protocol_version: 'DRAFT-2025-v3')`,
which adds extra burden. In this PR, Draft is made the default in order to provide the following benefits:

- Users are likely to prefer using the Draft version over the latest stable version
- No need to specify the `protocol_version` option when initializing `MCP::Server`
- Users generally do not need to know the internal development value `DRAFT-2025-v3`

The primary case for specifying `protocol_version` explicitly is when strict adherence to a particular version is required.
However, it seems more practical to prioritize use cases where backward-compatible features, including those from the Draft,
are desirable. This is the rationale behind making the Draft version the default.

In addition, making the Draft version the default protocol version preserves backward compatibility for `MCP::Server`.

NOTE: `DRAFT-2025-v3` is a development code, so its value cannot be set externally.
It is applied either when `protocol_version` is omitted or when `nil` is set explicitly,
such as with `MCP::Configuration.new(protocol_version: nil)`.
This prevents users from creating implementations that depend on development codes like `DRAFT-2025-v3`.

lib/mcp/configuration.rb

@@ -2,8 +2,10 @@
 
 module MCP
   class Configuration
-    DEFAULT_PROTOCOL_VERSION = "2025-06-18"
-    SUPPORTED_PROTOCOL_VERSIONS = [DEFAULT_PROTOCOL_VERSION, "2025-03-26", "2024-11-05"]
+    # DRAFT-2025-v3 is the latest draft protocol version:
+    # https://github.com/modelcontextprotocol/modelcontextprotocol/blob/14ec41c/schema/draft/schema.ts#L15
+    DRAFT_PROTOCOL_VERSION = "DRAFT-2025-v3"
+    SUPPORTED_STABLE_PROTOCOL_VERSIONS = ["2025-06-18", "2025-03-26", "2024-11-05"]
 
     attr_writer :exception_reporter, :instrumentation_callback
 
@@ -33,7 +35,7 @@ module MCP
     end
 
     def protocol_version
-      @protocol_version || DEFAULT_PROTOCOL_VERSION
+      @protocol_version || DRAFT_PROTOCOL_VERSION
     end
 
     def protocol_version?
@@ -93,8 +95,8 @@ module MCP
     private
 
     def validate_protocol_version!(protocol_version)
-      unless SUPPORTED_PROTOCOL_VERSIONS.include?(protocol_version)
-        message = "protocol_version must be #{SUPPORTED_PROTOCOL_VERSIONS[0...-1].join(", ")}, or #{SUPPORTED_PROTOCOL_VERSIONS[-1]}"
+      unless SUPPORTED_STABLE_PROTOCOL_VERSIONS.include?(protocol_version)
+        message = "protocol_version must be #{SUPPORTED_STABLE_PROTOCOL_VERSIONS[0...-1].join(", ")}, or #{SUPPORTED_STABLE_PROTOCOL_VERSIONS[-1]}"
         raise ArgumentError, message
       end
     end

lib/mcp/server.rb

@@ -106,6 +106,8 @@ module MCP
     def define_prompt(name: nil, title: nil, description: nil, arguments: [], &block)
       prompt = Prompt.define(name:, title:, description:, arguments:, &block)
       @prompts[prompt.name_value] = prompt
+
+      validate!
     end
 
     def define_custom_method(method_name:, &block)
@@ -171,6 +173,21 @@ module MCP
     private
 
     def validate!
+      # NOTE: The draft protocol version is the next version after 2025-03-26.
+      if @configuration.protocol_version <= "2025-03-26"
+        if server_info.key?(:title)
+          message = "Error occurred in server_info. `title` is not supported in protocol version 2025-03-26 or earlier"
+          raise ArgumentError, message
+        end
+
+        primitive_titles = [@tools.values, @prompts.values, @resources, @resource_templates].flatten.map(&:title)
+
+        if primitive_titles.any?
+          message = "Error occurred in #{primitive_titles.join(", ")}. `title` is not supported in protocol version 2025-03-26 or earlier"
+          raise ArgumentError, message
+        end
+      end
+
       if @configuration.protocol_version == "2024-11-05"
         if @instructions
           message = "`instructions` supported by protocol version 2025-03-26 or higher"