Enforce configured network proxy in codex sandbox (#27035)

## Why

`codex sandbox` can start a network proxy from a configured permission
profile. Previously, sandbox-level containment was tied to managed
network requirements rather than whether a proxy was actually active.
This meant config-driven proxy policies were not consistently enforced
as the sandbox's only network path.

## What changed

- Enable proxy-only network containment whenever `codex sandbox` starts
a network proxy.
- Apply the same active-proxy check to the macOS and Linux sandbox
paths.
- Add a Linux regression test that verifies a sandboxed command cannot
establish a direct connection while the configured proxy is active.

## Test plan

- `just test -p codex-cli debug_sandbox::tests`
- `sandbox_with_network_proxy_blocks_direct_loopback_access` runs on
Linux to cover the config-driven proxy path end to end.

codex-rs/sandboxing/src/landlock.rs

@@ -47,7 +47,8 @@ pub fn create_linux_sandbox_command_args_for_permission_profile(
         "--permission-profile".to_string(),
         permission_profile_json,
     ];
-    if use_legacy_landlock {
+    // Proxy-only networking requires bubblewrap's isolated network namespace.
+    if use_legacy_landlock && !allow_network_for_proxy {
         linux_cmd.push("--use-legacy-landlock".to_string());
     }
     if allow_network_for_proxy {
@@ -83,7 +84,8 @@ fn create_linux_sandbox_command_args(
         "--command-cwd".to_string(),
         command_cwd,
     ];
-    if use_legacy_landlock {
+    // Proxy-only networking requires bubblewrap's isolated network namespace.
+    if use_legacy_landlock && !allow_network_for_proxy {
         linux_cmd.push("--use-legacy-landlock".to_string());
     }
     if allow_network_for_proxy {

codex-rs/sandboxing/src/landlock_tests.rs

@@ -33,14 +33,16 @@ fn legacy_landlock_flag_is_included_when_requested() {
 }
 
 #[test]
-fn proxy_flag_is_included_when_requested() {
+fn proxy_flag_takes_precedence_over_legacy_landlock() {
     let command = vec!["/bin/true".to_string()];
     let command_cwd = Path::new("/tmp/link");
     let cwd = Path::new("/tmp");
+    let permission_profile = PermissionProfile::read_only();
 
-    let args = create_linux_sandbox_command_args(
+    let args = create_linux_sandbox_command_args_for_permission_profile(
         command,
         command_cwd,
+        &permission_profile,
         cwd,
         /*use_legacy_landlock*/ true,
         /*allow_network_for_proxy*/ true,
@@ -49,6 +51,7 @@ fn proxy_flag_is_included_when_requested() {
         args.contains(&"--allow-network-for-proxy".to_string()),
         true
     );
+    assert_eq!(args.contains(&"--use-legacy-landlock".to_string()), false);
 }
 
 #[test]

codex-rs/sandboxing/src/manager.rs

@@ -338,8 +338,8 @@ fn ensure_linux_bubblewrap_is_supported(
     allow_network_for_proxy: bool,
     is_wsl1: bool,
 ) -> Result<(), SandboxTransformError> {
-    let requires_bubblewrap = !use_legacy_landlock
-        && (!file_system_sandbox_policy.has_full_disk_write_access() || allow_network_for_proxy);
+    let requires_bubblewrap = allow_network_for_proxy
+        || (!use_legacy_landlock && !file_system_sandbox_policy.has_full_disk_write_access());
     if is_wsl1 && requires_bubblewrap {
         return Err(SandboxTransformError::Wsl1UnsupportedForBubblewrap);
     }

codex-rs/sandboxing/src/manager_tests.rs

@@ -342,6 +342,15 @@ fn wsl1_rejects_linux_bubblewrap_path() {
         ),
         Err(super::SandboxTransformError::Wsl1UnsupportedForBubblewrap)
     ));
+    assert!(matches!(
+        super::ensure_linux_bubblewrap_is_supported(
+            &FileSystemSandboxPolicy::unrestricted(),
+            /*use_legacy_landlock*/ true,
+            /*allow_network_for_proxy*/ true,
+            /*is_wsl1*/ true,
+        ),
+        Err(super::SandboxTransformError::Wsl1UnsupportedForBubblewrap)
+    ));
 }
 
 #[cfg(target_os = "linux")]