Methods `get_node` and `complete` were typed as returning a dynamically created unions `InvocationsUnion` and `InvocationOutputsUnion`, respectively.
Static type analysers cannot work with dynamic objects, so these methods end up as effectively un-annotated, returning `Unknown`.
They now return `BaseInvocation` and `BaseInvocationOutput`, respectively, which are the superclasses of all members of each union. This gives us the best type annotation that is possible.
Note: the return types of these methods are never introspected, so it doesn't really matter what they are at runtime.
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