fix: do not generate SDK-side requestId when server omits x-request-id header

When the response carries no x-request-id header, set requestID to null
instead of fabricating a UUID with an SDK-Generated- prefix. A null value
is the honest signal to callers that no server request ID is available.
Also removes the now-unused UUID import.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Devesh-Skyflow

src/main/java/com/skyflow/utils/HttpUtility.java

@@ -13,7 +13,6 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
-import java.util.UUID;
 
 public final class HttpUtility {
 
@@ -74,11 +73,7 @@ public static String sendRequest(String method, URL url, JsonObject params, Map<
 
             int httpCode = connection.getResponseCode();
             String requestID = connection.getHeaderField("x-request-id");
-            if (requestID != null) {
-                HttpUtility.requestID = requestID.split(",")[0];
-            } else {
-                HttpUtility.requestID = Constants.HttpUtilityExtra.SDK_GENERATED_PREFIX + UUID.randomUUID();
-            }
+            HttpUtility.requestID = requestID != null ? requestID.split(",")[0] : null;
             Map<String, List<String>> responseHeaders = connection.getHeaderFields();
             Reader streamReader;
             if (httpCode > 299) {