feat: retain NetworkProxy, when appropriate (#11207)

As of this PR, `SessionServices` retains a
`Option<StartedNetworkProxy>`, if appropriate.

Now the `network` field on `Config` is `Option<NetworkProxySpec>`
instead of `Option<NetworkProxy>`.

Over in `Session::new()`, we invoke `NetworkProxySpec::start_proxy()` to
create the `StartedNetworkProxy`, which is a new struct that retains the
`NetworkProxy` as well as the `NetworkProxyHandle`. (Note that `Drop` is
implemented for `NetworkProxyHandle` to ensure the proxies are shutdown
when it is dropped.)

The `NetworkProxy` from the `StartedNetworkProxy` is threaded through to
the appropriate places.


---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/11207).
* #11285
* __->__ #11207

codex-rs/cli/src/debug_sandbox.rs

@@ -130,10 +130,7 @@ async fn run_command_under_sandbox(
     let sandbox_policy_cwd = cwd.clone();
 
     let stdio_policy = StdioPolicy::Inherit;
-    let mut env = create_env(&config.shell_environment_policy, None);
-    if let Some(network) = config.network.as_ref() {
-        network.apply_to_env(&mut env);
-    }
+    let env = create_env(&config.shell_environment_policy, None);
 
     // Special-case Windows sandbox: execute and exit the process to emulate inherited stdio.
     if let SandboxType::Windows = sandbox_type {
@@ -216,6 +213,19 @@ async fn run_command_under_sandbox(
     #[cfg(not(target_os = "macos"))]
     let _ = log_denials;
 
+    // This proxy should only live for the lifetime of the child process.
+    let network_proxy = match config.network.as_ref() {
+        Some(spec) => Some(
+            spec.start_proxy()
+                .await
+                .map_err(|err| anyhow::anyhow!("failed to start managed network proxy: {err}"))?,
+        ),
+        None => None,
+    };
+    let network = network_proxy
+        .as_ref()
+        .map(codex_core::config::StartedNetworkProxy::proxy);
+
     let mut child = match sandbox_type {
         #[cfg(target_os = "macos")]
         SandboxType::Seatbelt => {
@@ -225,7 +235,7 @@ async fn run_command_under_sandbox(
                 config.sandbox_policy.get(),
                 sandbox_policy_cwd.as_path(),
                 stdio_policy,
-                None,
+                network.as_ref(),
                 env,
             )
             .await?
@@ -245,7 +255,7 @@ async fn run_command_under_sandbox(
                 sandbox_policy_cwd.as_path(),
                 use_bwrap_sandbox,
                 stdio_policy,
-                None,
+                network.as_ref(),
                 env,
             )
             .await?