Support customizing the Faraday client in MCP::Client::HTTP

[kstevens715]

Problem

MCP::Client::HTTP builds its Faraday connection internally in a private client method with no way to customize the middleware stack or adapter. This makes it difficult to add observability (request/response recording, failure logging) or swap the HTTP adapter without overriding a private method.

We're using the gem as an MCP client in a Rails app and need to inject Faraday middleware for tracing HTTP requests. Currently we subclass MCP::Client::HTTP and override client, which works but couples us to a private API.

Proposal

Accept an optional block in MCP::Client::HTTP.new that yields the Faraday builder, similar to how Faraday.new itself works:

transport = MCP::Client::HTTP.new(url: "https://example.com/mcp") do |faraday|
  faraday.use MyApp::Middleware::HttpRecorder
  faraday.adapter :typhoeus
end

The block would be called during Faraday client construction, after the default middleware (JSON request/response, raise_error, headers) is configured. This preserves the defaults while giving consumers a hook for additions.

Current workaround

class MyTransport < MCP::Client::HTTP
  private

  def client
    require_faraday!
    @client ||= Faraday.new url do |f|
      f.request :json
      f.response :json
      f.response :raise_error

      f.headers['Accept'] = ACCEPT_HEADER
      headers.each { |key, value| f.headers[key] = value }

      f.use MyApp::Middleware::HttpRecorder
      f.adapter :typhoeus
    end
  end
end

This duplicates the gem's default Faraday setup and will break if the gem changes its internal configuration.