Support running MCP conformance tests against the Ruby SDK

## Motivation

To assess and track the Ruby SDK's compliance with the MCP
specification, integrate `@modelcontextprotocol/conformance` as the
standard evaluation harness. This makes it possible to measure
conformance objectively, catch regressions early, and produce the kind
of evaluation data needed for SDK tier assessments.

## Scope

Only server conformance is included at this point. Client conformance
requires protocol-level features that MCP::Client does not yet
support (initialize handshake, notifications, ping, etc.).

## Usage

Run conformance tests:

```console
bundle exec rake conformance
```

Start the conformance server in the foreground for iterating on a single
scenario without the per-run startup overhead:

```console
bundle exec rake conformance_server
```

## Default Rake Task (`bundle exec rake`)

It is configured to run in order of lower execution cost: `rubocop`, `test`, and then `conformance`.

## CI

`.github/workflows/conformance.yml` file has been added to run the conformance tests in CI.

## SDK Tier Report

The MCP SDK Tier system (https://modelcontextprotocol.io/community/sdk-tiers)
requires SDK maintainers to self-assess and report results to the SDK Working
Group via https://github.com/modelcontextprotocol/modelcontextprotocol/issues.

To generate a full tier assessment report, use the `/mcp-sdk-tier-audit`
slash command from the https://github.com/modelcontextprotocol/conformance repository with
the conformance server running:

```console
# Terminal 1 (this repository): start the conformance server
bundle exec rake conformance_server

# Terminal 2 (conformance repository): run the tier audit skill as a slash command in Claude Code
/mcp-sdk-tier-audit /path/to/modelcontextprotocol/ruby-sdk http://localhost:9292/mcp
```

The skill evaluates conformance pass rate, issue label taxonomy, triage
metrics, documentation coverage, and policy compliance, then produces a
markdown report suitable for tier advancement submissions.

conformance/expected_failures.yml

@@ -0,0 +1,9 @@
+server:
+  # TODO: Server-to-client requests (sampling/createMessage, elicitation/create) are not implemented.
+  # `Transport#send_request` does not exist in the current SDK.
+  - tools-call-sampling
+  - tools-call-elicitation
+  - elicitation-sep1034-defaults
+  - elicitation-sep1330-enums
+  # TODO: The SDK does not extract `_meta.progressToken` from tool call requests or deliver `notifications/progress` to tools.
+  - tools-call-with-progress