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/tui/src/chatwidget/tests.rs

@@ -164,6 +164,7 @@ async fn resumed_initial_messages_render_history() {
                 message: "assistant reply".to_string(),
             }),
         ]),
+        network_proxy: None,
         rollout_path: Some(rollout_file.path().to_path_buf()),
     };
 
@@ -226,6 +227,7 @@ async fn replayed_user_message_preserves_text_elements_and_local_images() {
             text_elements: text_elements.clone(),
             local_images: local_images.clone(),
         })]),
+        network_proxy: None,
         rollout_path: Some(rollout_file.path().to_path_buf()),
     };
 
@@ -338,6 +340,7 @@ async fn submission_preserves_text_elements_and_local_images() {
         history_log_id: 0,
         history_entry_count: 0,
         initial_messages: None,
+        network_proxy: None,
         rollout_path: Some(rollout_file.path().to_path_buf()),
     };
     chat.handle_codex_event(Event {
@@ -417,6 +420,7 @@ async fn submission_prefers_selected_duplicate_skill_path() {
         history_log_id: 0,
         history_entry_count: 0,
         initial_messages: None,
+        network_proxy: None,
         rollout_path: Some(rollout_file.path().to_path_buf()),
     };
     chat.handle_codex_event(Event {
@@ -1108,6 +1112,7 @@ async fn make_chatwidget_manual(
         feedback_audience: FeedbackAudience::External,
         current_rollout_path: None,
         current_cwd: None,
+        session_network_proxy: None,
         status_line_invalid_items_warned: Arc::new(AtomicBool::new(false)),
         status_line_branch: None,
         status_line_branch_cwd: None,
@@ -2973,6 +2978,7 @@ async fn plan_slash_command_with_args_submits_prompt_in_plan_mode() {
         history_log_id: 0,
         history_entry_count: 0,
         initial_messages: None,
+        network_proxy: None,
         rollout_path: None,
     };
     chat.handle_codex_event(Event {