Revert "chore: sanbox extraction" (#4626)

Reverts openai/codex#4286

codex-rs/core/tests/suite/seatbelt.rs

@@ -169,12 +169,6 @@ async fn python_getpwuid_works_under_seatbelt() {
         return;
     }
 
-    // For local dev.
-    if which::which("python3").is_err() {
-        eprintln!("python3 not found in PATH, skipping test.");
-        return;
-    }
-
     // ReadOnly is sufficient here since we are only exercising user lookup.
     let policy = SandboxPolicy::ReadOnly;
     let command_cwd = std::env::current_dir().expect("getcwd");