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`

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

codex-rs/exec-server/Cargo.toml

@@ -20,6 +20,7 @@ async-trait = { workspace = true }
 base64 = { workspace = true }
 clap = { workspace = true, features = ["derive"] }
 codex-app-server-protocol = { workspace = true }
+codex-protocol = { workspace = true }
 codex-utils-absolute-path = { workspace = true }
 codex-utils-pty = { workspace = true }
 futures = { workspace = true }