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/expected_failures.yml

@@ -1 +1,28 @@
 server: []
+client:
+  # TODO: SSE reconnection not implemented in Ruby client.
+  - sse-retry
+  # TODO: Elicitation not implemented in Ruby client.
+  - elicitation-sep1034-client-defaults
+  # TODO: OAuth/auth not implemented in Ruby client.
+  - auth/metadata-default
+  - auth/metadata-var1
+  - auth/metadata-var2
+  - auth/metadata-var3
+  - auth/basic-cimd
+  - auth/scope-from-www-authenticate
+  - auth/scope-from-scopes-supported
+  - auth/scope-omitted-when-undefined
+  - auth/scope-step-up
+  - auth/scope-retry-limit
+  - auth/token-endpoint-auth-basic
+  - auth/token-endpoint-auth-post
+  - auth/token-endpoint-auth-none
+  - auth/pre-registration
+  - auth/2025-03-26-oauth-metadata-backcompat
+  - auth/2025-03-26-oauth-endpoint-fallback
+  - auth/client-credentials-jwt
+  - auth/client-credentials-basic
+  - auth/cross-app-access-complete-flow
+  - auth/offline-access-scope
+  - auth/offline-access-not-supported