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/app-server/src/codex_message_processor.rs

@@ -1634,13 +1634,30 @@ impl CodexMessageProcessor {
         let timeout_ms = params
             .timeout_ms
             .and_then(|timeout_ms| u64::try_from(timeout_ms).ok());
+        let started_network_proxy = match self.config.network.as_ref() {
+            Some(spec) => match spec.start_proxy().await {
+                Ok(started) => Some(started),
+                Err(err) => {
+                    let error = JSONRPCErrorError {
+                        code: INTERNAL_ERROR_CODE,
+                        message: format!("failed to start managed network proxy: {err}"),
+                        data: None,
+                    };
+                    self.outgoing.send_error(request, error).await;
+                    return;
+                }
+            },
+            None => None,
+        };
         let windows_sandbox_level = WindowsSandboxLevel::from_config(&self.config);
         let exec_params = ExecParams {
             command: params.command,
             cwd,
             expiration: timeout_ms.into(),
             env,
-            network: self.config.network.clone(),
+            network: started_network_proxy
+                .as_ref()
+                .map(codex_core::config::StartedNetworkProxy::proxy),
             sandbox_permissions: SandboxPermissions::UseDefault,
             windows_sandbox_level,
             justification: None,
@@ -1668,9 +1685,11 @@ impl CodexMessageProcessor {
         let outgoing = self.outgoing.clone();
         let request_for_task = request;
         let sandbox_cwd = self.config.cwd.clone();
+        let started_network_proxy_for_task = started_network_proxy;
         let use_linux_sandbox_bwrap = self.config.features.enabled(Feature::UseLinuxSandboxBwrap);
 
         tokio::spawn(async move {
+            let _started_network_proxy = started_network_proxy_for_task;
             match codex_core::exec::process_exec_tool_call(
                 exec_params,
                 &effective_policy,