Add instructions support to MCP::Server

The response of the `init` method can include `instructions`.
https://modelcontextprotocol.io/specification/2025-06-18/basic/lifecycle

`instructions` appear to be supported starting from the 2025-03-26 specification.

- https://modelcontextprotocol.io/specification/2025-03-26/basic/lifecycle (supports `instructions`)
- https://modelcontextprotocol.io/specification/2024-11-05/basic/lifecycle (does not support `instructions`)

If `instructions` is used with an unsupported protocol version, an `ArgumentError` will be raised.

As additional context, the TypeScript SDK also treats `instructions` as optional.
https://github.com/modelcontextprotocol/typescript-sdk/blob/1.16.0/src/types.ts#L356-L361

README.md

@@ -155,11 +155,11 @@ You can use the `Server#handle_json` method to handle requests.
 
 ```ruby
 class ApplicationController < ActionController::Base
-
   def index
     server = MCP::Server.new(
       name: "my_server",
       version: "1.0.0",
+      instructions: "Use the tools of this server as a last resort",
       tools: [SomeTool, AnotherTool],
       prompts: [MyPrompt],
       server_context: { user_id: current_user.id },