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.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+# Conformance test client for the MCP Ruby SDK.
+# Invoked by the conformance runner:
+#   MCP_CONFORMANCE_SCENARIO=<scenario> bundle exec ruby conformance/client.rb <server-url>
+#
+# The server URL is passed as the last positional argument.
+# The scenario name is read from the MCP_CONFORMANCE_SCENARIO environment variable,
+# which is set automatically by the conformance test runner.
+
+require_relative "../lib/mcp"
+
+scenario = ENV["MCP_CONFORMANCE_SCENARIO"]
+server_url = ARGV.last
+
+unless scenario && server_url
+  abort("Usage: MCP_CONFORMANCE_SCENARIO=<scenario> ruby conformance/client.rb <server-url>")
+end
+
+transport = MCP::Client::HTTP.new(url: server_url)
+client = MCP::Client.new(transport: transport)
+client.connect(client_info: { name: "ruby-sdk-conformance-client", version: MCP::VERSION })
+
+case scenario
+when "initialize"
+  client.tools
+when "tools_call"
+  tools = client.tools
+  add_numbers = tools.find { |t| t.name == "add_numbers" }
+  abort("Tool add_numbers not found") unless add_numbers
+  client.call_tool(tool: add_numbers, arguments: { a: 1, b: 2 })
+else
+  abort("Unknown or unsupported scenario: #{scenario}")
+end