Move all contextual validation code into its own function

[teor2345]

Motivation

The contextual validation function is hard to test.

It also directly accesses the sled state, but we want to work on sled and contextual validation at the same time.

Solution

Move the contextual validation function into the check module, and call it via a wrapper in the state service.

This change has two benefits:

• reduces conflicts with the sled refactor and any replacement
• allows the function to be called independently for testing

The code in this pull request has:

• Documentation Comments
• Unit Tests and Property Tests
  • This code change is a basic refactor which enables future tests

Review

@yaahc is also working on a sled refactor right now, they might conflict.

Follow Up Work

Do the rest of the difficulty contextual validation #802