Add Ruby MCP client example

## Motivation and Context

Add an LLM-powered chatbot MCP client example using the MCP Ruby SDK and the Anthropic Ruby SDK,
following the same pattern as other language examples.
https://github.com/modelcontextprotocol/ruby-sdk

The Gemfile specifies mcp version 0.9.1 or higher, which supports the stdio client.
https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.9.1

## Additional context

Once https://github.com/anthropics/anthropic-sdk-ruby/pull/161 is merged and released,
`gem "base64"` will no longer be needed in the Gemfile.

mcp-client-ruby/.env.example

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+ANTHROPIC_API_KEY=

mcp-client-ruby/Gemfile

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+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+gem "anthropic"
+gem "base64"
+gem "dotenv"
+gem "mcp", '>= 0.9.1'

mcp-client-ruby/README.md

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+# An LLM-Powered Chatbot MCP Client written in Ruby
+
+See the [Building MCP clients](https://modelcontextprotocol.io/tutorials/building-a-client) tutorial for more information.

mcp-client-ruby/client.rb

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+# frozen_string_literal: true
+
+require "anthropic"
+require "dotenv/load"
+require "json"
+require "mcp"
+
+class MCPClient
+  ANTHROPIC_MODEL = "claude-sonnet-4-5"
+
+  def initialize
+    @mcp_client = nil
+    @transport = nil
+    @anthropic_client = nil
+  end
+
+  def connect_to_server(server_script_path)
+    command = case File.extname(server_script_path)
+    when ".rb"
+      "ruby"
+    when ".py"
+      "python3"
+    when ".js"
+      "node"
+    else
+      raise ArgumentError, "Server script must be a .rb, .py, or .js file."
+    end
+
+    @transport = MCP::Client::Stdio.new(command: command, args: [server_script_path])
+    @mcp_client = MCP::Client.new(transport: @transport)
+
+    tool_names = @mcp_client.tools.map(&:name)
+    puts "\nConnected to server with tools: #{tool_names}"
+  end
+
+  def chat_loop
+    puts <<~MESSAGE
+      MCP Client Started!
+      Type your queries or 'quit' to exit.
+    MESSAGE
+
+    loop do
+      print "\nQuery: "
+      line = $stdin.gets
+      break if line.nil?
+
+      query = line.chomp.strip
+      break if query.downcase == "quit"
+      next if query.empty?
+
+      begin
+        response = process_query(query)
+        puts "\n#{response}"
+      rescue => e
+        puts "\nError: #{e.message}"
+      end
+    end
+  end
+
+  def cleanup
+    @transport&.close
+  end
+
+  private
+
+  def process_query(query)
+    messages = [{ role: "user", content: query }]
+
+    available_tools = @mcp_client.tools.map do |tool|
+      { name: tool.name, description: tool.description, input_schema: tool.input_schema }
+    end
+
+    # Initial Claude API call.
+    response = chat(messages, tools: available_tools)
+
+    # Process response and handle tool calls.
+    if response.content.any?(Anthropic::Models::ToolUseBlock)
+      assistant_content = response.content.filter_map do |content_block|
+        case content_block
+        when Anthropic::Models::TextBlock
+          { type: "text", text: content_block.text }
+        when Anthropic::Models::ToolUseBlock
+          { type: "tool_use", id: content_block.id, name: content_block.name, input: content_block.input }
+        end
+      end
+      messages << { role: "assistant", content: assistant_content }
+    end
+
+    response.content.each_with_object([]) do |content, response_parts|
+      case content
+      when Anthropic::Models::TextBlock
+        response_parts << content.text
+      when Anthropic::Models::ToolUseBlock
+        # Execute tool call via MCP.
+        result = @mcp_client.call_tool(name: content.name, arguments: content.input)
+        response_parts << "[Calling tool #{content.name} with args #{content.input.to_json}]"
+
+        tool_result_content = result.dig("result", "content")
+        result_text = if tool_result_content.is_a?(Array)
+          tool_result_content.filter_map { |content_item| content_item["text"] }.join("\n")
+        else
+          tool_result_content.to_s
+        end
+
+        messages << {
+          role: "user",
+          content: [{
+            type: "tool_result",
+            tool_use_id: content.id,
+            content: result_text
+          }]
+        }
+
+        # Get next response from Claude.
+        response = chat(messages)
+
+        response.content.each do |content_block|
+          response_parts << content_block.text if content_block.is_a?(Anthropic::Models::TextBlock)
+        end
+      end
+    end.join("\n")
+  end
+
+  def chat(messages, tools: nil)
+    params = { model: ANTHROPIC_MODEL, max_tokens: 1000, messages: messages }
+    params[:tools] = tools if tools
+
+    anthropic_client.messages.create(**params)
+  end
+
+  def anthropic_client
+    @anthropic_client ||= Anthropic::Client.new(api_key: ENV["ANTHROPIC_API_KEY"])
+  end
+end
+
+if ARGV.empty?
+  puts "Usage: ruby client.rb <path_to_server_script>"
+  exit 1
+end
+
+client = MCPClient.new
+
+begin
+  client.connect_to_server(ARGV[0])
+
+  api_key = ENV["ANTHROPIC_API_KEY"]
+  if api_key.nil? || api_key.empty?
+    puts <<~MESSAGE
+      No ANTHROPIC_API_KEY found. To query these tools with Claude, set your API key:
+        export ANTHROPIC_API_KEY=your-api-key-here
+    MESSAGE
+    exit
+  end
+
+  client.chat_loop
+rescue => e
+  puts "Error: #{e.message}"
+  exit 1
+ensure
+  client.cleanup
+end