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)

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

codex-rs/app-server-client/src/lib.rs

@@ -44,6 +44,7 @@ use codex_core::config::Config;
 use codex_core::config_loader::CloudRequirementsLoader;
 use codex_core::config_loader::LoaderOverrides;
 pub use codex_exec_server::EnvironmentManager;
+pub use codex_exec_server::ExecServerRuntimePaths;
 use codex_feedback::CodexFeedback;
 use codex_protocol::protocol::SessionSource;
 use serde::de::DeserializeOwned;