Support ping client API per MCP specification

[koic]

Motivation and Context

The MCP specification defines the ping utility for verifying that the peer of a connection is still responsive. The Ruby SDK server side already replies to incoming ping requests with an empty result, but MCP::Client had no public API to send a ping request. Users had to build the JSON-RPC payload themselves and call the transport directly.

This adds a client-side counterpart to the existing server handler and aligns the Ruby SDK in spirit with the Python and TypeScript SDKs, which expose ClientSession.send_ping and Client.ping respectively. Those SDKs validate the response envelope via type schemas (EmptyResult / EmptyResultSchema) and raise their validation library's generic exception (Pydantic's ValidationError / Zod's ZodError). The Ruby client applies a Hash type check on result (matching Python's behavior of requiring a dict while allowing arbitrary contents) and raises a new MCP::Client::ValidationError for malformed responses, keeping MCP::Client::ServerError reserved for JSON-RPC error responses.

How Has This Been Tested?

Client tests cover:

• request structure (method is ping, no params)
• empty result on success
• ServerError on JSON-RPC error response
• ValidationError on a response missing result
• ValidationError on a result of the wrong type (e.g., String)
• propagation of transport-level errors raised by transport.send_request

The existing server-side ping handler tests continue to pass unchanged.

Breaking Changes

None. MCP::Client#ping and MCP::Client::ValidationError are purely additive. The server-side ping handler behavior is unchanged.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed