fix: add null check for type_i field in OperationDeserializer

src/main/java/org/stellar/sdk/responses/gson/OperationDeserializer.java

@@ -27,9 +27,15 @@ public OperationResponse deserialize(
             .registerTypeAdapter(Predicate.class, new PredicateDeserializer())
             .create();
 
-    int type = json.getAsJsonObject().get("type_i").getAsInt();
+    JsonElement typeElement = json.getAsJsonObject().get("type_i");
+    if (typeElement == null || typeElement.isJsonNull()) {
+      throw new JsonParseException(
+          "Missing required field 'type_i'. Your Horizon version may be outdated.");
+    }
+    int type = typeElement.getAsInt();
     if (type < 0 || type >= AllOperationTypes.length) {
-      throw new IllegalArgumentException("Invalid operation type");
+      throw new JsonParseException(
+          "Unknown operation type: " + type + ". Your SDK version may be outdated.");
     }
 
     switch (AllOperationTypes[type]) {
@@ -88,7 +94,7 @@ public OperationResponse deserialize(
       case RESTORE_FOOTPRINT:
         return gson.fromJson(json, RestoreFootprintOperationResponse.class);
       default:
-        throw new IllegalArgumentException("Invalid operation type");
+        throw new AssertionError("Unhandled operation type: " + AllOperationTypes[type]);
     }
   }
 }

src/test/java/org/stellar/sdk/responses/OperationResponseTest.java

@@ -3,8 +3,10 @@
 import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 
+import com.google.gson.JsonParseException;
 import java.io.IOException;
 import java.math.BigInteger;
 import java.nio.charset.StandardCharsets;
@@ -703,4 +705,26 @@ public void testSetTrustLineFlagsOperation() throws IOException {
     assertArrayEquals(new Integer[] {4}, response.getClearFlags().toArray());
     assertArrayEquals(new String[] {"clawback_enabled"}, response.getClearFlagStrings().toArray());
   }
+
+  @Test
+  public void testDeserializeOperationWithMissingTypeI() {
+    String json = "{\"id\": \"12345\", \"type\": \"payment\"}";
+    JsonParseException exception =
+        assertThrows(
+            JsonParseException.class,
+            () -> GsonSingleton.getInstance().fromJson(json, OperationResponse.class));
+    assertTrue(exception.getMessage().contains("Missing required field 'type_i'"));
+    assertTrue(exception.getMessage().contains("Horizon version may be outdated"));
+  }
+
+  @Test
+  public void testDeserializeOperationWithUnknownTypeI() {
+    String json = "{\"id\": \"12345\", \"type\": \"unknown\", \"type_i\": 9999}";
+    JsonParseException exception =
+        assertThrows(
+            JsonParseException.class,
+            () -> GsonSingleton.getInstance().fromJson(json, OperationResponse.class));
+    assertTrue(exception.getMessage().contains("Unknown operation type: 9999"));
+    assertTrue(exception.getMessage().contains("SDK version may be outdated"));
+  }
 }