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/client_runner.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+# Runs `npx @modelcontextprotocol/conformance client` against the conformance client script.
+require "English"
+
+module Conformance
+  class ClientRunner
+    def initialize(scenario: nil, spec_version: nil, verbose: false)
+      @scenario = scenario
+      @spec_version = spec_version
+      @verbose = verbose
+    end
+
+    def run
+      command = build_command
+      puts "Command: #{command.join(" ")}\n\n"
+
+      system(*command)
+      conformance_exit_code = $CHILD_STATUS.exitstatus
+      exit(conformance_exit_code || 1) unless conformance_exit_code == 0
+    end
+
+    private
+
+    def build_command
+      expected_failures_yml = File.expand_path("expected_failures.yml", __dir__)
+      client_script = File.expand_path("client.rb", __dir__)
+
+      npx_command = [
+        "npx",
+        "--yes",
+        "@modelcontextprotocol/conformance",
+        "client",
+        "--command",
+        "bundle exec ruby #{client_script}",
+      ]
+      npx_command += if @scenario
+        ["--scenario", @scenario]
+      else
+        ["--suite", "all"]
+      end
+      npx_command += ["--spec-version", @spec_version] if @spec_version
+      npx_command += ["--verbose"] if @verbose
+      npx_command += ["--expected-failures", expected_failures_yml]
+      npx_command
+    end
+  end
+end