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.


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[//]: # (BEGIN SAPLING FOOTER)
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* #11285
* __->__ #11207

codex-rs/core/src/state/service.rs

@@ -5,6 +5,7 @@ use crate::RolloutRecorder;
 use crate::agent::AgentControl;
 use crate::analytics_client::AnalyticsEventsClient;
 use crate::client::ModelClient;
+use crate::config::StartedNetworkProxy;
 use crate::exec_policy::ExecPolicyManager;
 use crate::file_watcher::FileWatcher;
 use crate::hooks::Hooks;
@@ -38,6 +39,7 @@ pub(crate) struct SessionServices {
     pub(crate) skills_manager: Arc<SkillsManager>,
     pub(crate) file_watcher: Arc<FileWatcher>,
     pub(crate) agent_control: AgentControl,
+    pub(crate) network_proxy: Option<StartedNetworkProxy>,
     pub(crate) state_db: Option<StateDbHandle>,
     /// Session-scoped model client shared across turns.
     pub(crate) model_client: ModelClient,