Backend: Refactor BaseInput#validate!

Summary

The following discussion from !132574 (merged) should be addressed:

• @avielle started a discussion:

Proposal

It's a bit difficult to follow what's happening in this method 🤔 It should be easier to add validations as we go forward. I plan to refactor this method in a follow-up

Additional details

Some relevant technical details, if applicable, such as:

• Does this need a feature flag?
• Does there need to be an associated instrumentation issue created related to this work?
• Is there an example response showing the data structure that should be returned (new endpoints only)?
• What permissions should be used?
• Is this EE or CE?
  • EE
  • CE
• Additional comments:

Implementation Table

Group	Issue Link
backend	👈 You are here
backend	#393401 (closed)

Links/References