Update Streamable HTTP client example to use MCP::Client

[atesgoral]

Summary

• rewrite the Streamable HTTP client example to use MCP::Client for initialize, tools/list, and tools/call
• keep raw Net::HTTP only for the optional GET SSE stream, which the SDK transport does not implement yet
• adapt the superseded Add MCP Streamable HTTP specification support for the client #210 example rewrite to the current client API by reading session state from the HTTP transport and handling nil 202 ACK responses

Verification

• ruby -c examples/streamable_http_client.rb
• bundle exec rubocop --cache false examples/streamable_http_client.rb
• ruby -I lib -e 'require "mcp"; require_relative "examples/streamable_http_client"; puts :loaded'
• smoke-tested against examples/streamable_http_server.rb with scripted input