Add MCP client conformance regression gates (#36810)

## What changed

- Add a harness that runs the Codex executable against a pinned official MCP
  client conformance suite across shipping, intermediate, and modern protocol
  versions, HTTP and stdio transports, and OAuth scenarios.
- Add a separate app-server regression matrix for transport, security, schema,
  pagination, SSE, multi-round request, and catalog-boundary behavior.
- Check both suites against committed baselines so previously passing or
  required checks cannot disappear or newly fail, while keeping known failures
  visible in complete reports.
- Run the fixture self-tests and both executable-level gates from the Python
  and TypeScript SDK test workflows.

## Testing

- Add unit tests for the fixture server, official-suite adapter, conformance
  runner, baseline comparison, and reviewer regression runner.

GitOrigin-RevId: de59f039294e34ed72873d9f6940b52e89172c0d

pnpm-workspace.yaml

@@ -14,4 +14,6 @@ strictDepBuilds: true
 trustPolicy: no-downgrade
 trustPolicyIgnoreAfter: 10080
 trustPolicyExclude: []
-allowBuilds: {}
+allowBuilds:
+  # Build only the official MCP conformance CLI pinned by the TypeScript SDK.
+  "@modelcontextprotocol/conformance": true