fix: avoid double serialization in VersionedAttestation SCALE size_hint

The size_hint impl was calling to_bytes() to compute the exact size,
then encode_to() would serialize again. Return 0 instead since SCALE's
size_hint is best-effort and used only for pre-allocation.

Author: kvinwang

dstack-attest/src/attestation.rs

@@ -355,7 +355,7 @@ pub enum VersionedAttestation {
 
 impl Encode for VersionedAttestation {
     fn size_hint(&self) -> usize {
-        self.to_bytes().map(|b| b.len()).unwrap_or(0)
+        0
     }
 
     fn encode_to<T: Output + ?Sized>(&self, dest: &mut T) {