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/exec/tests/event_processor_with_json_output.rs

@@ -98,6 +98,7 @@ fn session_configured_produces_thread_started_event() {
             history_log_id: 0,
             history_entry_count: 0,
             initial_messages: None,
+            network_proxy: None,
             rollout_path: Some(rollout_path),
         }),
     );