Support ping client API per MCP specification

## 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.

test/mcp/client_test.rb

@@ -538,6 +538,67 @@ module MCP
       refute(result["hasMore"])
     end
 
+    def test_ping_sends_request_and_returns_empty_hash
+      transport = mock
+      mock_response = { "result" => {} }
+
+      transport.expects(:send_request).with do |args|
+        req = args[:request]
+        req[:method] == "ping" &&
+          req[:jsonrpc] == "2.0" &&
+          !req.key?(:params)
+      end.returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      result = client.ping
+
+      assert_equal({}, result)
+    end
+
+    def test_ping_raises_server_error_on_error_response
+      transport = mock
+      mock_response = { "error" => { "code" => -32_603, "message" => "Internal error" } }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      error = assert_raises(Client::ServerError) { client.ping }
+      assert_equal(-32_603, error.code)
+      assert_equal("Internal error", error.message)
+    end
+
+    def test_ping_raises_validation_error_when_result_is_missing
+      transport = mock
+      mock_response = {}
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      error = assert_raises(Client::ValidationError) { client.ping }
+      assert_equal("Response validation failed: missing or invalid `result`", error.message)
+    end
+
+    def test_ping_raises_validation_error_when_result_is_wrong_type
+      transport = mock
+      mock_response = { "result" => "ok" }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      error = assert_raises(Client::ValidationError) { client.ping }
+      assert_equal("Response validation failed: missing or invalid `result`", error.message)
+    end
+
+    def test_ping_propagates_transport_errors
+      transport = mock
+      transport_error = StandardError.new("read timeout")
+      transport.expects(:send_request).raises(transport_error).once
+
+      client = Client.new(transport: transport)
+      error = assert_raises(StandardError) { client.ping }
+      assert_equal("read timeout", error.message)
+    end
+
     def test_tools_auto_paginates_across_multiple_pages
       transport = mock