Add assertions in TestCSHttpConnectionErrorHandling to verify non-null response and correct error message for network errors, enhancing error handling test coverage.

Author: harshithad0703

contentstack/src/test/java/com/contentstack/sdk/TestCSHttpConnectionErrorHandling.java

@@ -82,5 +82,7 @@ public void testGenerateBuiltErrorWithKnownType() throws Exception {
         invokeGenerateBuiltError(networkError);
 
         response = connection.getResponse();
+        assertNotNull(response);
+        assertEquals("NetworkError", response.optString("error_message"));
     }
 }