## Why
Concurrent tests can inherit writable descriptors for executable fixtures, causing `ETXTBSY` when those fixtures launch on Linux.
## What changed
Expose shared `write_executable` and `copy_executable` helpers from `codex_utils_cargo_bin` and adopt them in CLI, exec-server, and MCP tests. On Linux, writes and copies complete in separate processes so sibling test spawns cannot inherit the writable descriptors. Script fixtures receive mode `0o755`, and copied executables retain their source permissions.
Remove the executable-busy retry loop from the program resolver test now that its script uses the shared helper.
GitOrigin-RevId: 61624158190bb1e75c27d9eb9c87a2defc147fb7