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/mcp-server/src/outgoing_message.rs

@@ -307,6 +307,7 @@ mod tests {
                 history_log_id: 1,
                 history_entry_count: 1000,
                 initial_messages: None,
+                network_proxy: None,
                 rollout_path: Some(rollout_file.path().to_path_buf()),
             }),
         };
@@ -348,6 +349,7 @@ mod tests {
             history_log_id: 1,
             history_entry_count: 1000,
             initial_messages: None,
+            network_proxy: None,
             rollout_path: Some(rollout_file.path().to_path_buf()),
         };
         let event = Event {
@@ -413,6 +415,7 @@ mod tests {
             history_log_id: 1,
             history_entry_count: 1000,
             initial_messages: None,
+            network_proxy: None,
             rollout_path: Some(rollout_file.path().to_path_buf()),
         };
         let event = Event {