Discussion: Should we allow converters that go to dict as well as converters that go to basemodels?

This is the current behaviour, but could cause confusion. See [comment](https://github.com/DiamondLightSource/daq-config-server/pull/129#discussion_r2588349695)

## Acceptance Criteria
- Decide and make changes if needed

Start by reading the linked discussion on pull request #129 and tracing the current converter behavior described there. Decide whether converters to dict and BaseModel should both be allowed; if changes are needed, update the relevant implementation and tests, but the issue names no files or tests.

Written by the indexing model from the issue text.