ESQL: declared MVs wrongly optimised past STATS

[bpintea]

Description

The following test should work:

evalGroupAfterStatsOverRow
ROW type = ["one", "two"]
| STATS BY type
| EVAL is_one = type == "one"
| EVAL is_two = type == "two"
;

type:keyword   |is_one:boolean |is_two:boolean
one            |true           |false
two            |false          |true

but if now fails, is_one and is_two being wrongly returned as null.
(Disabling PropagateEvalFoldables in the optimiser fixes it.)

[elasticsearchmachine]

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

[wchaparro]

@bpintea similar to #114322 ?

[bpintea]

@bpintea similar to #114322 ?

Yes, they are manifestations of the same cause. I'll keep this open, until we merge the fix, I'd like to make sure the test above will be part of the/a fix.