## Summary
- split the joined `PATH` before running system `bwrap` lookup
- keep the existing workspace-local `bwrap` skip behavior intact
- add regression tests that exercise real multi-entry search paths
## Why
The PATH-based lookup added in #15791 still wrapped the raw `PATH`
environment value as a single `PathBuf` before passing it through
`join_paths()`. On Unix, a normal multi-entry `PATH` contains `:`, so
that wrapper path is invalid as one path element and the lookup returns
`None`.
That made Codex behave as if no system `bwrap` was installed even when
`bwrap` was available on `PATH`, which is what users in #15340 were
still hitting on `0.117.0-alpha.25`.
## Impact
System `bwrap` discovery now works with normal multi-entry `PATH` values
instead of silently falling back to the vendored binary.
Fixes#15340.
## Validation
- `just fmt`
- `cargo test -p codex-sandboxing`
- `cargo test -p codex-linux-sandbox`
- `just fix -p codex-sandboxing`
- `just argument-comment-lint`
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