[sandbox] Filter out certain non-sandbox errors (#1804)

## Summary
Users frequently complain about re-approving commands that have failed
for non-sandbox reasons. We can't diagnose with complete accuracy which
errors happened because of a sandbox failure, but we can start to
eliminate some common simple cases.

This PR captures the most common case I've seen, which is a `command not
found` error.

## Testing
- [x] Added unit tests
- [x] Ran a few cases locally

codex-rs/core/src/lib.rs

@@ -38,7 +38,7 @@ pub mod plan_tool;
 mod project_doc;
 pub mod protocol;
 mod rollout;
-mod safety;
+pub(crate) mod safety;
 pub mod seatbelt;
 pub mod shell;
 pub mod spawn;
@@ -47,3 +47,4 @@ pub mod util;
 
 pub use apply_patch::CODEX_APPLY_PATCH_ARG1;
 pub use client_common::model_supports_reasoning_summaries;
+pub use safety::get_platform_sandbox;