v3 signature challenge does not match the defined type

This should be caught in our signer's validation, but we could at least be consistent here:
https://github.com/MetaMask/test-dapp/blob/main/src/index.js#L801

Start at src/index.js around line 801 and inspect the v3 signature challenge alongside the signer's validation and its defined type. Confirm where the mismatch is represented and verify that the challenge and type are consistent, using any existing validation or end-to-end checks you find.

Written by the indexing model from the issue text.