Reject trailing bytes in block proof payloads

[quby1845]

Summary

• decode block-proof payloads through a reader with an explicit byte budget
• reject bytes trailing the canonical BlockProof encoding
• add regression coverage for canonical and suffixed proof payloads

Motivation

Deserializable::read_from_bytes intentionally permits trailing input. In the proof writer this
means a canonical BlockProof followed by arbitrary bytes can pass deserialization and the entire
payload can then be persisted and re-broadcast.

This change makes decoding exact: it parses a proof within the supplied payload budget and verifies
that the reader is exhausted before the proof is accepted.

Scope

This is an interim hardening change, not cryptographic block-proof verification. BlockProof is
currently a placeholder in miden-protocol, and the protocol does not yet expose the proof data,
public inputs, prover output, and verifier API required to implement the full fix described in
#2383. The existing verification TODO remains, with that dependency documented in the code.

Testing

• canonical BlockProof encodings are accepted
• a canonical encoding followed by arbitrary bytes is rejected

Refs #2383.