## Why
A model-provided shell path should select the requested shell type without
allowing that path to determine which executable Codex runs.
## What changed
- Resolve model-provided shells through Codex's normal shell discovery and
fallback logic after detecting their type.
- Keep the configured packaged zsh executable when the zsh-fork feature is
enabled and the file exists.
- Update shell, unified exec, and network approval expectations to use the
resolved local executable and arguments.
GitOrigin-RevId: ebe6f7eec2cfd1c0548d5bf1a26b7a30dba02cc2