Consider fixing parsing of literals from objects

[hellovai]

Sometimes when an LLM responds to a primitive value, it can use an object to contain its response. We can benefit from parsing objects of singular keys as the values themslves and drop they key. we already do this in some places.

This new test fails:

// file: engine/baml-lib/jsonish/src/tests/test_literals.rs

test_deserializer!(
    test_union_literal_with_multiple_types_from_object,
    EMPTY_FILE,
    r#"{
  "status": 1
}"#,
    FieldType::Union(vec![
        FieldType::Literal(LiteralValue::Int(1)),
        FieldType::Literal(LiteralValue::Bool(true)),
        FieldType::Literal(LiteralValue::String("THREE".into())),
    ]),
    1
);