Add sandbox support to filesystem APIs (#16751)

## Summary
- add optional `sandboxPolicy` support to the app-server filesystem
request surface
- thread sandbox-aware filesystem options through app-server and
exec-server adapters
- enforce sandboxed read/write access in the filesystem abstraction with
focused local and remote coverage

## Validation
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-exec-server file_system`
- `cargo test -p codex-app-server suite::v2::fs`

---------

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

codex-rs/Cargo.lock

@@ -2094,6 +2094,7 @@ dependencies = [
  "base64 0.22.1",
  "clap",
  "codex-app-server-protocol",
+ "codex-protocol",
  "codex-utils-absolute-path",
  "codex-utils-cargo-bin",
  "codex-utils-pty",