Summary

A literal in the select list becomes a $project value. Rendered bare, MQL reads it as an
inclusion or exclusion flag, or as a field path, rather than as a constant, so it has to be wrapped
in $literal.

Hibernate ORM emits such literals on its own, not only when an application writes one. Loading an
entity whose embeddable holds the non-owning side of an association projects a null placeholder:

{"aggregate": "shelters", "pipeline": [
  {"$match": {"_id": {"$eq": 1}}},
  {"$project": {"#c_1": {"$literal": null}}}
]}

A numeric literal is the case MQL is most likely to misread rather than reject: {{select 1, i.name
from Item i}} renders a value of 1, which $project treats as an inclusion flag for a field named
after the alias.

Proposed fix

In AbstractMqlTranslator, wrap a projection expression that resolves to a value in
AstLiteralExpression before adding it to the $project stage.

Acceptance

select null, i.name from Item i, select 'foobar', i.name from Item i and {{select 1, i.name
from Item i}} each return the literal at position 1 and the attribute at position 2.