Based on eip-3668, both the ccip-read invocation function and its callback function MUST have the same return type.
In GatewayFetchTarget, the callback function (GatewayFetchTarget::fetchCallback) does not implement any explicit return type. Its understandable that GatewayFetchTarget is agnostic to the data fetched cross-chain and their data type. But in such cases, to be in full compliant with eip-3668, fetchCallback should explicitly return bytes memory which also allows further type interpretation downstream.
https://github.com/code-423n4/2024-12-unruggable/blob/9e0e908a207f088dd843fcdc3e4de5fa0edd6c5c/contracts/GatewayFetchTarget.sol#L41
Consider revising GatewayFetchTarget::fetchCallback to explicitly return bytes memory type.
