fix: improve detection of malformed input

fuzz/fuzz_targets/fuzz_encoding_roundtrip.rs

@@ -4,9 +4,14 @@
 //! Invariants exercised:
 //!
 //! 1. **Encode/decode stability** — `encode(decode(encode(tx))) == encode(tx)`.
-//! 2. **Canonical encoding** — if raw fuzzer bytes decode successfully, re-encoding must
+//! 2. **No-panic on adversarial bytes** — `from_bytes(raw_fuzz_data)` must never panic,
+//!    whether it returns `Ok` or `Err`. (Tests 3 & 4.)
+//! 3. **Canonical encoding** — if raw fuzzer bytes decode successfully, re-encoding must
 //!    reproduce those exact bytes (`encode(decode(data)) == data`). This catches non-canonical
 //!    encodings that are accepted by the decoder but silently normalised on the way out.
+//!    Because `borsh::from_slice` (used by `from_bytes`) consumes *all* bytes and errors on
+//!    trailing data, `Ok` implies every input byte was semantically meaningful, and the output
+//!    must be identical. (Tests 3 & 4, `Ok` branch.)
 //!
 //! `PrivacyPreservingTransaction` is excluded because its ZK receipt cannot be
 //! reconstructed in a fuzzing loop.
@@ -47,8 +52,11 @@ fuzz_target!(|data: &[u8]| {
         );
     }
 
-    // ── Test 3: Canonical encoding — raw bytes that decode must re-encode identically ──
+    // ── Test 3: No-panic decode + canonical encoding (PublicTransaction) ─────────────
-    if let Ok(tx) = PublicTransaction::from_bytes(data) {
+    // Invariant 2: from_bytes must never panic on any input — Ok or Err both valid, no panic.
+    // Invariant 3: on Ok, re-encoding must reproduce the original bytes exactly.
+    let pub_raw_result = PublicTransaction::from_bytes(data); // ← no-panic check (invariant 2)
+    if let Ok(tx) = pub_raw_result {
         let re_encoded = tx.to_bytes();
         assert_eq!(
             data,
@@ -58,8 +66,10 @@ fuzz_target!(|data: &[u8]| {
         );
     }
 
-    // ── Test 4: Canonical encoding for ProgramDeploymentTransaction ──────────────────
+    // ── Test 4: No-panic decode + canonical encoding (ProgramDeploymentTransaction) ──
-    if let Ok(tx) = ProgramDeploymentTransaction::from_bytes(data) {
+    // Same two invariants as Test 3, applied to ProgramDeploymentTransaction.
+    let prog_raw_result = ProgramDeploymentTransaction::from_bytes(data); // ← no-panic check
+    if let Ok(tx) = prog_raw_result {
         let re_encoded = tx.to_bytes();
         assert_eq!(
             data,