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/exec-server/src/fs_helper_main.rs

@@ -0,0 +1,45 @@
+use std::error::Error;
+
+use tokio::io;
+use tokio::io::AsyncReadExt;
+use tokio::io::AsyncWriteExt;
+
+use crate::fs_helper::FsHelperRequest;
+use crate::fs_helper::FsHelperResponse;
+use crate::fs_helper::run_direct_request;
+
+pub fn main() -> ! {
+    let exit_code = match tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+    {
+        Ok(runtime) => match runtime.block_on(run_main()) {
+            Ok(()) => 0,
+            Err(err) => {
+                eprintln!("fs sandbox helper failed: {err}");
+                1
+            }
+        },
+        Err(err) => {
+            eprintln!("failed to start fs sandbox helper runtime: {err}");
+            1
+        }
+    };
+    std::process::exit(exit_code);
+}
+
+async fn run_main() -> Result<(), Box<dyn Error + Send + Sync>> {
+    let mut input = Vec::new();
+    io::stdin().read_to_end(&mut input).await?;
+    let request: FsHelperRequest = serde_json::from_slice(&input)?;
+    let response = match run_direct_request(request).await {
+        Ok(payload) => FsHelperResponse::Ok(payload),
+        Err(error) => FsHelperResponse::Error(error),
+    };
+    let mut stdout = io::stdout();
+    stdout
+        .write_all(serde_json::to_string(&response)?.as_bytes())
+        .await?;
+    stdout.write_all(b"\n").await?;
+    Ok(())
+}