unit-tests to validate working of specialized flattening code

[de-sh]

          Can we add tests to better document the expectations we wish to be fulfilled by this code

Originally posted by @de-sh in https://github.com/parseablehq/parseable/pull/1196#pullrequestreview-2625652941

[riturajFi]

Hi @de-sh this issue seems to be open still. Would love to contribute