propagate sierra-emu VM failure as Error from ContractExecutor::Emu

The Emu arm previously called .expect() on VirtualMachine::run, which
aborts the host on any internal VM failure. The Aot arm propagates
errors via Result; align the Emu arm with it.

VirtualMachine::run returns Option<ContractExecutionResult> (None means
"VM never produced a final state"). Convert None to
Error::UnexpectedValue with a descriptive message so callers can decide
how to handle.

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

Author: avi-starkware

src/executor.rs

@@ -3,12 +3,12 @@
 //! This module provides methods to execute the programs, either via JIT or compiled ahead
 //! of time. It also provides a cache to avoid recompiling previously compiled programs.
 
+#[cfg(feature = "sierra-emu")]
+pub use self::contract_executor::EmuContractInfo;
 pub use self::{
     aot::AotNativeExecutor, contract::AotContractExecutor, contract_executor::ContractExecutor,
     jit::JitNativeExecutor,
 };
-#[cfg(feature = "sierra-emu")]
-pub use self::contract_executor::EmuContractInfo;
 use crate::{
     arch::{AbiArgument, ValueWithInfoWrapper},
     error::{panic::ToNativeAssertError, Error},

src/executor/contract_executor.rs

@@ -15,6 +15,8 @@ use starknet_types_core::felt::Felt;
 #[cfg(feature = "sierra-emu")]
 use std::sync::Arc;
 
+#[cfg(feature = "sierra-emu")]
+use crate::error::Error;
 use crate::error::Result;
 use crate::execution_result::ContractExecutionResult;
 use crate::executor::AotContractExecutor;
@@ -89,9 +91,11 @@ impl ContractExecutor {
 
                 virtual_machine.call_contract(selector, gas, args.to_vec(), emu_builtin_costs);
 
-                let result = virtual_machine
-                    .run(&mut syscall_handler)
-                    .expect("sierra-emu VM run failed");
+                // `VirtualMachine::run` returns `None` when the VM never produced a
+                // final state — propagate as an error rather than aborting the host.
+                let result = virtual_machine.run(&mut syscall_handler).ok_or_else(|| {
+                    Error::UnexpectedValue("sierra-emu VM produced no final state".to_string())
+                })?;
 
                 Ok(ContractExecutionResult {
                     remaining_gas: result.remaining_gas,