Some Schema rules are embedded in the code

All schema restrictions on YAML should be in the schema itself so that users can identify what the problem is using the schema aware vscode editor.

This is not the case for techui.yaml at least. For example this line of code adds a restriction that is not surfaced in the schema:

https://github.com/diamondlightsource/techui-builder/blob/bcf67389ecbaf982c17d1e041ccffe806b67adb2/src/techui_builder/models.py#L76

@coretl FYI