Add strict PallasMessage deserialization in challenge and fix existing tests

[jCabala]

Problem

We want BluePallas::challenge to stay strict: input must be serialized PallasMessage.
Current test vectors/helpers still pass raw bytes in some paths, so tests fail.

Fix

What needs to be done

1. Update test vectors to store/use serialized PallasMessage bytes, not raw payload bytes.
2. Update test helpers to stop fallback parsing and require serialized PallasMessage.

[scaraven]

An idea that we can do is feature gate the fall-through so that it only exists when we run tests. Otherwise, if we have a normal binary then the fall-through will panic. Worth considering