make StarknetSyscallHandler::cheatcode default return error felt

Default impl previously panicked with unimplemented!(). On the sierra-emu
path, a contract that invokes the `cheatcode` libfunc against a handler
that didn't override the method would unwind through the VM and abort
the host. Production handlers (e.g. the blockifier's NativeSyscallHandler)
don't implement cheatcode at all, so the default impl is what runs.

Soft-fail with a single error felt instead, so misuse manifests as a
normal syscall error rather than an unhandled panic. Test handlers that
implement cheatcode override the default and are unaffected.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: avi-starkware

cairo-native-syscalls/src/lib.rs

@@ -365,7 +365,11 @@ pub trait StarknetSyscallHandler {
         remaining_gas: &mut u64,
     ) -> SyscallResult<Vec<Felt>>;
 
+    /// Test-only Starknet syscall. Production handlers don't implement it; the default
+    /// returns a single error felt rather than panicking, so a malicious contract that
+    /// invokes `cheatcode` against a handler that didn't override the method can't crash
+    /// the host. Test handlers should override.
     fn cheatcode(&mut self, _selector: Felt, _input: &[Felt]) -> Vec<Felt> {
-        unimplemented!();
+        vec![Felt::from_bytes_be_slice(b"cheatcode unsupported")]
     }
 }