Support running MCP client conformance tests against the Ruby SDK

## Motivation and Context

PR #248 introduced server conformance testing using `@modelcontextprotocol/conformance`.
This extends coverage to the client side so that `MCP::Client` behavior can also be validated against
the MCP specification.

### Scope

Client conformance is driven by the conformance runner, which spawns test servers for each scenario
and invokes `conformance/client.rb` against them. The client script delegates the MCP initialize
handshake to `MCP::Client#connect` and executes scenario-specific operations (for example listing
tools or calling a tool).

### Usage

Run all conformance tests (server + client):

```console
bundle exec rake conformance
```

Run a single client scenario:

```console
bundle exec rake conformance SCENARIO=initialize
```

List available scenarios:

```console
bundle exec rake conformance_list
```

## How Has This Been Tested?

Ran `bundle exec rake conformance` locally.

- Server conformance: 40 passed, 0 failed.
- Client conformance: 3 passed, 54 failed (all 54 listed in `expected_failures.yml`).

## Breaking Change

No. This change only adds test infrastructure under `conformance/` and does not modify SDK behavior.

conformance/server_runner.rb

@@ -0,0 +1,101 @@
+# frozen_string_literal: true
+
+# Starts the conformance server and runs `npx @modelcontextprotocol/conformance` against it.
+require "English"
+require "net/http"
+require_relative "server"
+
+module Conformance
+  class ServerRunner
+    # Timeout for waiting for the Puma server to start.
+    SERVER_START_TIMEOUT = 20
+    SERVER_POLL_INTERVAL = 0.5
+    SERVER_HEALTH_CHECK_RETRIES = (SERVER_START_TIMEOUT / SERVER_POLL_INTERVAL).to_i
+
+    def initialize(port: Server::DEFAULT_PORT, scenario: nil, spec_version: nil, verbose: false)
+      @port = port
+      @scenario = scenario
+      @spec_version = spec_version
+      @verbose = verbose
+    end
+
+    def run
+      command = build_command
+      server_pid = start_server
+
+      run_conformance(command, server_pid: server_pid)
+    end
+
+    private
+
+    def build_command
+      expected_failures_yml = File.expand_path("expected_failures.yml", __dir__)
+
+      npx_command = ["npx", "--yes", "@modelcontextprotocol/conformance", "server", "--url", "http://localhost:#{@port}/mcp"]
+      npx_command += ["--scenario", @scenario] if @scenario
+      npx_command += ["--spec-version", @spec_version] if @spec_version
+      npx_command += ["--verbose"] if @verbose
+      npx_command += ["--expected-failures", expected_failures_yml]
+      npx_command
+    end
+
+    def start_server
+      puts "Starting conformance server on port #{@port}..."
+
+      server_pid = fork do
+        Conformance::Server.new(port: @port).start
+      end
+
+      health_url = URI("http://localhost:#{@port}/health")
+      ready = false
+      SERVER_HEALTH_CHECK_RETRIES.times do
+        begin
+          response = Net::HTTP.get_response(health_url)
+          if response.code == "200"
+            ready = true
+            break
+          end
+        rescue Errno::ECONNREFUSED, Errno::ECONNRESET, Net::ReadTimeout
+          # not ready yet
+        end
+        sleep(SERVER_POLL_INTERVAL)
+      end
+
+      unless ready
+        warn("ERROR: Conformance server did not start within #{SERVER_START_TIMEOUT} seconds")
+        terminate_server(server_pid)
+        exit(1)
+      end
+
+      puts "Server ready. Running conformance tests..."
+
+      server_pid
+    end
+
+    def run_conformance(command, server_pid:)
+      puts "Command: #{command.join(" ")}\n\n"
+
+      conformance_exit_code = nil
+      begin
+        system(*command)
+        conformance_exit_code = $CHILD_STATUS.exitstatus
+      ensure
+        terminate_server(server_pid)
+      end
+
+      exit(conformance_exit_code || 1) unless conformance_exit_code == 0
+    end
+
+    def terminate_server(pid)
+      Process.kill("TERM", pid)
+    rescue Errno::ESRCH
+      # process already exited
+    ensure
+      begin
+        Process.wait(pid)
+      rescue Errno::ECHILD
+        # already reaped
+      end
+    end
+  end
+end