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

test/mcp/server_test.rb

@@ -64,6 +64,7 @@ module MCP
       @server = Server.new(
         name: @server_name,
         version: "1.2.3",
+        instructions: "Optional instructions for the client",
         tools: [@tool, @tool_that_raises],
         prompts: [@prompt],
         resources: [@resource],
@@ -135,6 +136,7 @@ module MCP
             name: @server_name,
             version: "1.2.3",
           },
+          instructions: "Optional instructions for the client",
         },
       }
 
@@ -750,6 +752,33 @@ module MCP
       assert_equal Server::DEFAULT_VERSION, response[:result][:serverInfo][:version]
     end
 
+    test "server uses instructions when not configured" do
+      server = Server.new(name: "test_server")
+      request = {
+        jsonrpc: "2.0",
+        method: "initialize",
+        id: 1,
+      }
+
+      response = server.handle(request)
+      refute response[:result].key?(:instructions)
+    end
+
+    test "server uses instructions when configured with protocol version 2025-03-26" do
+      configuration = Configuration.new(protocol_version: "2025-03-26")
+      server = Server.new(name: "test_server", instructions: "The server instructions.", configuration: configuration)
+      assert_equal("The server instructions.", server.instructions)
+    end
+
+    test "raises error if instructions are used with protocol version 2024-11-05" do
+      configuration = Configuration.new(protocol_version: "2024-11-05")
+
+      exception = assert_raises(ArgumentError) do
+        Server.new(name: "test_server", instructions: "The server instructions.", configuration: configuration)
+      end
+      assert_equal("`instructions` supported by protocol version 2025-03-26 or higher", exception.message)
+    end
+
     test "#define_tool adds a tool to the server" do
       @server.define_tool(
         name: "defined_tool",