Improve testing for Runtime SDK variable discovery

This is an umbrella issue to follow up on improvements to testing and documenting #7985

- [ ] Add unit tests testing the variables returned from the test-extension
- [ ] Add an end to end test that covers complex variables
- [ ] Add an end to end test which covers moving from a set of inline variables and patches to external variables and patches
- [ ] Use runtimeSDK settings in a runtime variable discovery and/or patch call
- [ ] Add documentation on best practices for writing variable discovery extensions.

Start by reviewing issue #7985 and the test-extension mentioned here. Then map the requested unit and end-to-end coverage for complex variables, inline-to-external variables and patches, and runtimeSDK settings. Done means the relevant tests pass and the best-practices documentation is added.

Written by the indexing model from the issue text.