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/cli/tests/sandbox_network_proxy.rs

@@ -0,0 +1,70 @@
+#![cfg(target_os = "linux")]
+
+use std::net::TcpListener;
+
+use anyhow::Result;
+use tempfile::TempDir;
+
+const BWRAP_UNAVAILABLE_ERR: &str = "bubblewrap is unavailable";
+
+#[test]
+fn sandbox_with_network_proxy_blocks_direct_loopback_access() -> Result<()> {
+    let codex_home = TempDir::new()?;
+    let listener = TcpListener::bind("127.0.0.2:0")?;
+    let port = listener.local_addr()?.port();
+    std::fs::write(
+        codex_home.path().join("config.toml"),
+        r#"
+default_permissions = "network-test"
+
+[features]
+network_proxy = true
+use_legacy_landlock = true
+
+[permissions.network-test]
+extends = ":workspace"
+
+[permissions.network-test.network]
+enabled = true
+mode = "full"
+"#,
+    )?;
+
+    let url = format!("http://127.0.0.2:{port}/");
+    let output = std::process::Command::new(codex_utils_cargo_bin::cargo_bin("codex")?)
+        .env("CODEX_HOME", codex_home.path())
+        .args([
+            "sandbox",
+            "--permissions-profile",
+            "network-test",
+            "--",
+            "curl",
+            "--noproxy",
+            "*",
+            "--silent",
+            "--show-error",
+            "--connect-timeout",
+            "1",
+            "--max-time",
+            "2",
+            url.as_str(),
+        ])
+        .output()?;
+
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    if stderr.contains(BWRAP_UNAVAILABLE_ERR) {
+        eprintln!("skipping network proxy sandbox test: bubblewrap is unavailable");
+        return Ok(());
+    }
+
+    assert_eq!(
+        output.status.code(),
+        Some(7),
+        "expected direct loopback access to be blocked; status={:?}; stdout={}; stderr={}",
+        output.status.code(),
+        String::from_utf8_lossy(&output.stdout),
+        stderr,
+    );
+
+    Ok(())
+}