User-provided value is not used in create/upsert path for expression attributes.

[fractlrao]

If an attribute is defined as an expression (example below), the runtime ignores any user-provided value for create and upsert path. However, it uses the user-provided value, overriding the expression, for query+update path.

  entity Resource {
      ...
      FirstName String,
      PreferredFirstName String @optional,
      LastName String,
      FullName String @expr (helper.calculateFullName (FirstName, LastName, PreferredFirstName)),
      ...
  }

We need to handle these cases uniformly - override the expression value with any user-provided value in all cases.