## Why
Concurrent Linux test spawns can inherit writable descriptors for executable fixtures, preventing those fixtures from being executed even after the writing test closes its descriptor.
## What changed
Replace direct executable fixture writes and copies across the Rust tests with `codex_utils_cargo_bin::write_executable` and `copy_executable`. These existing helpers complete writes and copies in separate processes on Linux.
Remove the empty temporary inode before creating fake bubblewrap executables, and stage tampered bubblewrap bytes in a separate data file before copying them into the executable fixture. Add the required test dependencies.
GitOrigin-RevId: c875c83088731a4933ef1574e8d79d8cecc12f54
## Why
When a container denies fresh `/proc` mounts, the existing sandbox fallback retains the caller's `/proc` while creating a new PID namespace. Process IDs inside the sandbox can then differ from those exposed by `/proc`, breaking process lookups.
## What changed
- Add `codex exec-server --linux-sandbox-pid-namespace=inherit` to reuse the caller's PID namespace and `/proc` for both process and filesystem helpers. Repository config and command environment variables cannot enable it.
- Keep `isolate` as the default, preserving the existing mount fallback and compatibility with older helpers. Inheritance requires an updated helper and omits `--unshare-pid` and `--as-pid-1`.
- Preserve other sandbox restrictions. With `:minimal`, bind the inherited `/proc` read-only with its container masks and apply explicit filesystem denials afterward.
Inheritance is intended for dedicated environments: it allows sandboxed commands to signal other same-UID processes, including the executor.
## Testing
Add unit and integration coverage for namespace flags, denied proc mounts, consistent process IDs, retained sandbox restrictions, and startup-only selection across process and filesystem helpers.
GitOrigin-RevId: dd13f2b9286d7edcf366b3a42a455f4d78daaf27
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