[codex-rs] Improve linux sandbox timeouts (#662)

* Fixes flaking rust unit test
* Adds explicit sandbox exec timeout handling

codex-rs/core/src/error.rs

@@ -22,6 +22,14 @@ pub enum SandboxErr {
     #[error("seccomp backend error")]
     SeccompBackend(#[from] seccompiler::BackendError),
 
+    /// Command timed out
+    #[error("command timed out")]
+    Timeout,
+
+    /// Command was killed by a signal
+    #[error("command was killed by a signal")]
+    Signal(i32),
+
     /// Error from linux landlock
     #[error("Landlock was not able to fully enforce all sandbox rules")]
     LandlockRestrict,