feat(nodes): add validation for invoke method return types

[psychedelicious]

Summary

Adds some checks for invocations' implementations of the invoke method, raising an error when the method isn't defined correctly.

Related Issues / Discussions

See discussion here https://discord.com/channels/1020123559063990373/1233408559065206865

QA Instructions

@dunkeroni I expect this commit to prevent invoke from launching, erroring on the problematic node you are working on.

Merge Plan

n/a

Checklist

• [x] The PR has a short but descriptive title, suitable for a changelog
• [ ] Tests added / updated (if applicable)
• [x] Documentation added / updated (if applicable)