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/core/tests/suite/unified_exec.rs

@@ -191,7 +191,11 @@ async fn create_workspace_directory(
 ) -> Result<std::path::PathBuf> {
     let abs_path = test.config.cwd.join(rel_path.as_ref());
     test.fs()
-        .create_directory(&abs_path, CreateDirectoryOptions { recursive: true })
+        .create_directory(
+            &abs_path,
+            CreateDirectoryOptions { recursive: true },
+            /*sandbox*/ None,
+        )
         .await?;
     Ok(abs_path.into_path_buf())
 }