feat: experimental env var: CODEX_SANDBOX_NETWORK_DISABLED

Previous to this change:

```
$ cargo run --bin codex -- debug seatbelt --full-auto -- cargo test
---- keeps_previous_response_id_between_tasks stdout ----

thread 'keeps_previous_response_id_between_tasks' panicked at /Users/mbolin/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wiremock-0.6.3/src/mock_server/builder.rs:107:46:
Failed to bind an OS port for a mock server.: Os { code: 1, kind: PermissionDenied, message: "Operation not permitted" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    keeps_previous_response_id_between_tasks

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `-p codex-core --test previous_response_id`
```

codex-rs/cli/src/landlock.rs

@@ -3,6 +3,8 @@
 //! On Linux the command is executed inside a Landlock + seccomp sandbox by
 //! calling the low-level `exec_linux` helper from `codex_core::linux`.
 
+use codex_core::exec::StdioPolicy;
+use codex_core::exec::spawn_child;
 use codex_core::protocol::SandboxPolicy;
 use std::os::unix::process::ExitStatusExt;
 use std::process;
@@ -19,8 +21,9 @@ pub fn run_landlock(command: Vec<String>, sandbox_policy: SandboxPolicy) -> anyh
     // Spawn a new thread and apply the sandbox policies there.
     let handle = std::thread::spawn(move || -> anyhow::Result<ExitStatus> {
         let cwd = std::env::current_dir()?;
-        codex_core::linux::apply_sandbox_policy_to_current_thread(sandbox_policy, &cwd)?;
-        let status = Command::new(&command[0]).args(&command[1..]).status()?;
+        codex_core::linux::apply_sandbox_policy_to_current_thread(&sandbox_policy, &cwd)?;
+        let child = spawn_child(command, cwd, sandbox_policy, StdioPolicy::Inherit)?;
+        let status = child.status()?;
         Ok(status)
     });
     let status = handle

codex-rs/cli/src/seatbelt.rs

@@ -1,4 +1,5 @@
-use codex_core::exec::create_seatbelt_command;
+use codex_core::exec::StdioPolicy;
+use codex_core::exec::spawn_command_under_seatbelt;
 use codex_core::protocol::SandboxPolicy;
 
 pub async fn run_seatbelt(
@@ -6,10 +7,9 @@ pub async fn run_seatbelt(
     sandbox_policy: SandboxPolicy,
 ) -> anyhow::Result<()> {
     let cwd = std::env::current_dir().expect("failed to get cwd");
-    let seatbelt_command = create_seatbelt_command(command, &sandbox_policy, &cwd);
-    let status = tokio::process::Command::new(seatbelt_command[0].clone())
-        .args(&seatbelt_command[1..])
-        .spawn()
+    let child =
+        spawn_command_under_seatbelt(command, &sandbox_policy, cwd, StdioPolicy::Inherit).await;
+    let status = child
         .map_err(|e| anyhow::anyhow!("Failed to spawn command: {}", e))?
         .wait()
         .await