AttributeDefinition validateOperation should convert all expression strings to ModelType.EXPRESSION

AttributeDefinition.validateOperation currently only converts a string to an expression if the attribute supports expressions. It should convert any time it sees the syntax. This way if a STRING attribute that doesn't support expressions finds one, it will reject it instead of accepting it as a raw string.

Scheduling for 7.3 out of concern this may break things in 7.2 without time to notice/adapt.