Test README protocol version examples

This revealed that the `protocol_version` accessors weren't available on
the `Server` at all.

Author: sambostock

README.md

@@ -242,12 +242,14 @@ config.instrumentation_callback = ->(data) {
 
 The server's protocol version can be overridden using the `protocol_version` class method:
 
+<!-- SNIPPET ID: set_server_protocol_version -->
 ```ruby
 MCP::Server.protocol_version = "2024-11-05"
 ```
 
 This will make all new server instances use the specified protocol version instead of the default version. The protocol version can be reset to the default by setting it to `nil`:
 
+<!-- SNIPPET ID: unset_server_protocol_version -->
 ```ruby
 MCP::Server.protocol_version = nil
 ```

lib/mcp/server.rb

@@ -9,6 +9,14 @@ module MCP
   class Server
     DEFAULT_VERSION = "0.1.0"
 
+    class << self
+      def protocol_version = MCP.configuration.protocol_version
+
+      def protocol_version=(version)
+        MCP.configuration.protocol_version = version
+      end
+    end
+
     class RequestHandlerError < StandardError
       attr_reader :error_type
       attr_reader :original_error

test/fixtures/files/code_snippet_wrappers/readme/set_server_protocol_version.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+require "mcp"
+
+require_relative "code_snippet"
+
+puts MCP::Server.protocol_version

test/fixtures/files/code_snippet_wrappers/readme/unset_server_protocol_version.rb

@@ -0,0 +1 @@
+set_server_protocol_version.rb

test/integration/readme_code_snippets_test.rb

@@ -132,6 +132,11 @@ class ReadmeCodeSnippetsTest < ActiveSupport::TestCase
     STDOUT
   end
 
+  test "Protocol Version example works exactly as documented in README" do
+    assert_equal("2024-11-05",                                 run_code_snippet("set_server_protocol_version").chomp)
+    assert_equal(MCP::Configuration::DEFAULT_PROTOCOL_VERSION, run_code_snippet("unset_server_protocol_version").chomp)
+  end
+
   private
 
   def assert_json_lines(expected, actual, message = "Expected the given JSON lines")