Refactor trueup check spec

What does this MR do and why?

• Add error type to allow better verification in spec
  • This is to ensure later changes wouldn't accidentally alter the expected validation error type.
• Fix example of when "when trueup quantity with threshold is equal to the required quantity"

How to set up and validate locally

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #361345