[ESQL] Testing for date nanos union type

[not-napoleon]

#111850 and #112150 add in conversion functions that can be used as union types for date nanos, but they don't have much in the way of testing for the union type use case. This ticket is for adding that testing.

In particular, we need to make sure that the union type correctly pushes down to lucene. That will, functionally, mean sending two different long filters to lucene, since the backing longs are of different units.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)