Expand test coverage and sync completed coverage with spec.

+review REVIEW-5177
Add explicit test for inputs being finalised when used in dsl rules.

+review REVIEW-5177
Add explicit test for supporting duplicate input definitions in DSL rule.

+review REVIEW-5177
Mark off completed test coverage that is implied through other tests.

i.e. doesn't quite warrant its own test.

+review REVIEW-5177
Improve error message when invalid model path is used as rule input reference, and add test coverage.

+review REVIEW-5177
remove unused.

+review REVIEW-5177
Add some notes on extracting all rule information at runtime vs. compile time.

+review REVIEW-5177
Add explicit coverage of use of model {} block in script plugin with input extraction.

+review REVIEW-5177