Proto,tx_rollup: Consider the size of the message in the rejection

Context

There is an issue in a corner case of the rejection operation of TORU that basically makes it possible for an attacker to publish a message for which one cannot provide a “large enough” proof, and that can be used by the same attacker to publish a wrongful commitment that cannot be refuted. This is a deal breaker for the TORU feature as a whole, but it does not endanger the safety of the layer-1.

This simple fix addresses the issue.

Manually testing the MR

Checklist

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