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/apply-patch/src/standalone_executable.rs

@@ -71,6 +71,7 @@ pub fn run_main() -> i32 {
         &mut stdout,
         &mut stderr,
         codex_exec_server::LOCAL_FS.as_ref(),
+        /*sandbox*/ None,
     )) {
         Ok(()) => {
             // Flush to ensure output ordering when used in pipelines.