chore: add toBuilder to new test definition

spi/control-plane/transfer-spi/src/testFixtures/java/org/eclipse/dataspaceconnector/transfer/store/TestFunctions.java

@@ -111,7 +111,12 @@ public static TransferProcess initialTransferProcess(String processId, String da
     @JsonTypeName("dataspaceconnector:testresourcedef")
     @JsonDeserialize(builder = TestResourceDef.Builder.class)
     public static class TestResourceDef extends ResourceDefinition {
-
+
+        @Override
+        public <RD extends ResourceDefinition, B extends ResourceDefinition.Builder<RD, B>> B toBuilder() {
+            return null;
+        }
+
         @JsonPOJOBuilder(withPrefix = "")
         public static class Builder extends ResourceDefinition.Builder<TestResourceDef, Builder> {
             private Builder() {