feat(form-logic): introduce form logic model validation

[yong-jie]

Problem

See #2208

In form logic, specific field types are only compatible with certain specific "condition states", e.g. dropdown fields can only be used with "is equals to" or "is either" conditions.

However, the backend does not enforce this coupling between field types and condition states. It simply accepts any field ID linked to any possible condition state. For example, nothing is stopping the frontend from mistakenly saving a logic unit where an "is greater than" condition is applied to a dropdown field. This introduces risks for the React frontend migration, since sending API calls with the wrong logic unit shape could result in corruption of the database state.

Solution

1. Refactor createFormLogic and updateFormLogic to use .save(), because mongoose does not grant access to the underlying form document for the update operations.

2. Move LogicMap from being a frontend logic to a shared util.

3. Add validators on the form_logics array

Breaking Changes

• No - this PR is backwards compatible

Features:

Form logic now does validation on the field type and state. This ensures that things like yes/no fields don't have invalid condition states like less than.

Manual tests

• Via API, attempt to save mismatched form logic (e.g. logic state ''is less than or equal to' for a dropdown field) and check that model validation disallows it.
• For each valid logic field type (dropdown, radio, number, yes/no, rating, decimal), create a logic unit involving that field type to make sure valid logic is not affected.

[mantariksh]

main concern is a regression where we are disallowing logic referencing non-existent fields

[mantariksh]

final request - could you pls help add tests in form.server.model.spec.ts to ensure that the logic validation works as expected? in particular under the 'Base Schema' suite, it would be good to add a test to ensure that we do not accidentally disallow logic which references non-existent fields

had another thought

[mantariksh]

I just realised there's another logic condition field called ifValueType. do we need to check that these are valid for each fieldType too?

[yong-jie]

ifValueType is a frontend-only value that's for some reason being tracked in the database. this is a tech debt, but afaik no validation is needed/possible