Add client-level connect handshake to stdio transport

## Motivation and Context

Follow-up to https://github.com/modelcontextprotocol/ruby-sdk/pull/336

mcp gem 0.15.0 introduced an explicit `MCP::Client#connect` method for the stdio transport,
aligning with the Python and TypeScript SDKs which require explicit handshake calls regardless
of transport type:

- Python SDK: `ClientSession.initialize()`
  https://github.com/modelcontextprotocol/python-sdk
- TypeScript SDK: `Client.connect(transport)`
  https://github.com/modelcontextprotocol/typescript-sdk

This sample is updated to call `@mcp_client.connect` explicitly after creating the client,
demonstrating the recommended initialization pattern. The Gemfile constraint is bumped to `>= 0.15.0`
to require the new API.

## How Has This Been Tested?

Ran the sample against `weather-server-ruby` and confirmed the connection, tool listing,
and chat loop continue to work as expected.

## Breaking Changes

None. The change updates the sample to use the new explicit `connect` API,
which was added as a non-breaking addition in mcp gem 0.15.0.

mcp-client-ruby/Gemfile

@@ -5,4 +5,4 @@ source "https://rubygems.org"
 gem "anthropic"
 gem "base64"
 gem "dotenv"
-gem "mcp", '>= 0.9.1'
+gem "mcp", '>= 0.15.0'

mcp-client-ruby/client.rb

@@ -28,6 +28,7 @@ class MCPClient
 
     @transport = MCP::Client::Stdio.new(command: command, args: [server_script_path])
     @mcp_client = MCP::Client.new(transport: @transport)
+    @mcp_client.connect
 
     tool_names = @mcp_client.tools.map(&:name)
     puts "\nConnected to server with tools: #{tool_names}"