feat: implement ping request handler in MCP async client (#203)

- Add ping request handler that responds with empty Map instead of null
- Ensure compliance with MCP protocol requirement for ping responses
- Add test for ping message request handling

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

mcp/src/main/java/io/modelcontextprotocol/client/McpAsyncClient.java

@@ -187,6 +187,9 @@ public class McpAsyncClient {
 		// Request Handlers
 		Map<String, RequestHandler<?>> requestHandlers = new HashMap<>();
 
+		// Ping MUST respond with an empty data, but not NULL response.
+		requestHandlers.put(McpSchema.METHOD_PING, params -> Mono.just(Map.of()));
+
 		// Roots List Request Handler
 		if (this.clientCapabilities.roots() != null) {
 			requestHandlers.put(McpSchema.METHOD_ROOTS_LIST, rootsListRequestHandler());

mcp/src/test/java/io/modelcontextprotocol/client/McpAsyncClientResponseHandlerTests.java

@@ -525,4 +525,30 @@ class McpAsyncClientResponseHandlerTests {
 			.hasMessage("Elicitation handler must not be null when client capabilities include elicitation");
 	}
 
+	@Test
+	void testPingMessageRequestHandling() {
+		MockMcpClientTransport transport = initializationEnabledTransport();
+
+		McpAsyncClient asyncMcpClient = McpClient.async(transport).build();
+
+		assertThat(asyncMcpClient.initialize().block()).isNotNull();
+
+		// Simulate incoming ping request from server
+		McpSchema.JSONRPCRequest pingRequest = new McpSchema.JSONRPCRequest(McpSchema.JSONRPC_VERSION,
+				McpSchema.METHOD_PING, "ping-id", null);
+		transport.simulateIncomingMessage(pingRequest);
+
+		// Verify response
+		McpSchema.JSONRPCMessage sentMessage = transport.getLastSentMessage();
+		assertThat(sentMessage).isInstanceOf(McpSchema.JSONRPCResponse.class);
+
+		McpSchema.JSONRPCResponse response = (McpSchema.JSONRPCResponse) sentMessage;
+		assertThat(response.id()).isEqualTo("ping-id");
+		assertThat(response.error()).isNull();
+		assertThat(response.result()).isInstanceOf(Map.class);
+		assertThat(((Map<?, ?>) response.result())).isEmpty();
+
+		asyncMcpClient.closeGracefully();
+	}
+
 }