Formalize our supported (provider) interface

Our supported provider is not really documented at all. We need to add docs to what API exactly we support, and enforce it with tests.

Must include:
- Accepted param types
- Tests for accepted param types

We should also throw errors on non-accepted param types, to ensure devs don't think things are working when they aren't.

Locate the supported provider interface and any existing provider tests, then read how provider parameters are currently handled. Document the accepted parameter types, add tests for them, and verify that unsupported types produce errors; the issue is complete when the interface behavior is explicit and enforced.

Written by the indexing model from the issue text.