fix(core) rm execute_exec_request sandbox_policy (#16422)

## Summary
In #11871 we started consolidating on ExecRequest.sandbox_policy instead
of passing in a separate policy object that theoretically could differ
(but did not). This finishes the some parameter cleanup.

This should be a simple noop, since all 3 callsites of this function
already used a cloned object from the ExecRequest value.

## Testing
- [x] Existing tests pass

codex-rs/core/src/tasks/user_shell.rs

@@ -185,14 +185,9 @@ pub(crate) async fn execute_user_shell_command(
         tx_event: session.get_tx_event(),
     });
 
-    let exec_result = execute_exec_request(
-        exec_env,
-        &sandbox_policy,
-        stdout_stream,
-        /*after_spawn*/ None,
-    )
-    .or_cancel(&cancellation_token)
-    .await;
+    let exec_result = execute_exec_request(exec_env, stdout_stream, /*after_spawn*/ None)
+        .or_cancel(&cancellation_token)
+        .await;
 
     match exec_result {
         Err(CancelErr::Cancelled) => {