feat(completion): implement completion API support (#141)

Add completion API support to the MCP protocol implementation:

- Add CompleteRequest and CompleteResult schema classes
- Implement completion capabilities in ServerCapabilities
- Add completion handlers in McpAsyncServer and McpServer
- Add completion client methods in McpAsyncClient and McpSyncClient
- Add CompletionRefKey and completion specifications in McpServerFeatures
- Add integration test for completion functionality
- Fix isPresent() check to use isEmpty() in WebMvcSseServerTransportProvider
- Replace McpServerFeatures.CompletionRefKey by McpSchemaCompleteReference

Co-authored-by: Christian Tzolov <christian.tzolov@broadcom.com>

Signed-off-by: jitokim <pigberger70@gmail.com>

mcp-spring/mcp-spring-webmvc/src/main/java/io/modelcontextprotocol/server/transport/WebMvcSseServerTransportProvider.java

@@ -300,7 +300,7 @@ public class WebMvcSseServerTransportProvider implements McpServerTransportProvi
 			return ServerResponse.status(HttpStatus.SERVICE_UNAVAILABLE).body("Server is shutting down");
 		}
 
-		if (!request.param("sessionId").isPresent()) {
+		if (request.param("sessionId").isEmpty()) {
 			return ServerResponse.badRequest().body(new McpError("Session ID missing in message endpoint"));
 		}