fix: prevent public class fields from shadowing association values

[ephys]

Pull Request Checklist

• Have you added new tests to prevent regressions?
• If a documentation update is necessary, have you opened a PR to the documentation repository?
• Did you update the typescript typings accordingly (if applicable)?
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Description Of Change

Eager-loaded associations were suffering from the same issue as attributes: they get shadowed by public class fields declared on the model.

This adds a workaround. The actual solution would be to move everything from the constructor inside of the build method, so everything can be done after the class properties have been defined instead of before.

This will be possible to do in v8 when we ban using "new" on the model and require always using the build static method instead (deprecation warning & TODO are already present)