Run exec-server fs operations through sandbox helper (#17294)

## Summary
- run exec-server filesystem RPCs requiring sandboxing through a
`codex-fs` arg0 helper over stdin/stdout
- keep direct local filesystem execution for `DangerFullAccess` and
external sandbox policies
- remove the standalone exec-server binary path in favor of top-level
arg0 dispatch/runtime paths
- add sandbox escape regression coverage for local and remote filesystem
paths

## Validation
- `just fmt`
- `git diff --check`
- remote devbox: `cd codex-rs && bazel test --bes_backend=
--bes_results_url= //codex-rs/exec-server:all` (6/6 passed)

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/Cargo.lock

@@ -2097,9 +2097,9 @@ dependencies = [
  "arc-swap",
  "async-trait",
  "base64 0.22.1",
- "clap",
  "codex-app-server-protocol",
  "codex-protocol",
+ "codex-sandboxing",
  "codex-utils-absolute-path",
  "codex-utils-cargo-bin",
  "codex-utils-pty",