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/README.md

@@ -0,0 +1,103 @@
+# MCP Conformance Tests
+
+Validates the Ruby SDK's conformance to the MCP specification using [`@modelcontextprotocol/conformance`](https://github.com/modelcontextprotocol/conformance).
+
+## Prerequisites
+
+- Node.js (for `npx`)
+- `bundle install` completed
+
+## Running the Tests
+
+### Run all scenarios
+
+```bash
+bundle exec rake conformance
+```
+
+Starts the conformance server, runs all active scenarios against it, prints a pass/fail
+summary for each scenario, and exits with a non-zero status code if any unexpected failures
+are detected. Scenarios listed in `expected_failures.yml` are allowed to fail without
+affecting the exit code.
+
+### Environment variables
+
+| Variable       | Description                          | Default |
+|----------------|--------------------------------------|---------|
+| `PORT`         | Server port                          | `9292`  |
+| `SCENARIO`     | Run a single scenario by name        | (all)   |
+| `SPEC_VERSION` | Filter scenarios by spec version     | (all)   |
+| `VERBOSE`      | Show raw JSON output when set        | (off)   |
+
+```bash
+# Run a single scenario
+bundle exec rake conformance SCENARIO=ping
+
+# Use a different port with verbose output
+bundle exec rake conformance PORT=3000 VERBOSE=1
+
+# Start the server on a specific port
+bundle exec rake conformance_server PORT=3000
+```
+
+### Start the server and test separately
+
+```bash
+# Terminal 1: start the server
+bundle exec rake conformance_server
+
+# Terminal 2: run all scenarios
+npx @modelcontextprotocol/conformance server --url http://localhost:9292/mcp
+
+# Terminal 2: run a single scenario
+npx @modelcontextprotocol/conformance server --url http://localhost:9292/mcp --scenario ping
+```
+
+Keeps the server alive between test runs, which avoids the startup overhead when iterating
+on a single scenario. Stop the server with Ctrl+C when done.
+
+### List available scenarios
+
+```bash
+bundle exec rake conformance_list
+```
+
+Prints all scenario names that can be passed to `SCENARIO`.
+
+## 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
+[modelcontextprotocol/modelcontextprotocol issues](https://github.com/modelcontextprotocol/modelcontextprotocol/issues).
+
+To generate a full tier assessment report, use the `/mcp-sdk-tier-audit` slash command from
+the [modelcontextprotocol/conformance](https://github.com/modelcontextprotocol/conformance)
+repository with the conformance server running:
+
+```bash
+# 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.
+
+## File Structure
+
+```
+conformance/
+  server.rb              # Conformance server (Rack + Puma, default port 9292)
+  runner.rb              # Starts the server, runs npx conformance, exits with result code
+  expected_failures.yml  # Baseline of known-failing scenarios
+  README.md              # This file
+```
+
+## Known Limitations
+
+Known-failing scenarios are registered in `conformance/expected_failures.yml`. They are allowed to
+fail without affecting the exit code and are tracked to catch regressions.
+These are shown in the output of `bundle exec rake conformance`.