Remove `raw-message-fallback` feature guard

[jCabala]

Remove raw-message-fallback feature guard

Problem

challenge()'s raw-byte fallback is currently behind the raw-message-fallback cargo feature (on for tests, off in prod). It's a stopgap — cargo features are additive, so any future crate enabling it would silently re-weaken production. We want the fallback gone entirely and prod/test behavior identical.

Solution

Delete the fallback branch (always reject non-PallasMessage input) and the feature, then update tests to sign real PallasMessages:

• interoperability_tests.rs: wrap b"Hello from the Raspberry Devs" in PallasMessage::from_parts(...).serialize().
• mina_compatibility.rs::frost_even_commitment: wrap b"Test message for FROST even commitment" the same way.
• helpers/mod.rs: drop the fallback in verify_signature.
• vectors.json / vectors-big-identifier.json: re-encode "message" as a PallasMessage and regenerate the dependent signature fields.
• The 5 frost_core::tests::{ciphersuite_generic,refresh}::* tests (check_sign_with_dealer, check_sign_with_dkg, check_sign_with_dealer_and_identifiers, check_refresh_shares_with_dealer, check_refresh_shares_with_dkg) hardcode "message to sign" inside the dependency; reimplement them locally with frost-core's public primitives, signing a valid PallasMessage.