fix(messaging): incorrect SNS notification parsing

[havenbarnes]

Problem

We don't seem to be parsing single-SNS event callbacks properly. Here's a real example from Loki:

Changes

Fix parsing

How did you test this code?

Added unit tests for parsing behavior so we have can start to form a baseline understanding of SNS webhook body shape

Changelog: (features only) Is this feature complete?

[greptile-apps]

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