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)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/11207).
* #11285
* __->__ #11207

codex-rs/core/src/tasks/user_shell.rs

@@ -147,7 +147,7 @@ pub(crate) async fn execute_user_shell_command(
             &turn_context.shell_environment_policy,
             Some(session.conversation_id),
         ),
-        network: turn_context.config.network.clone(),
+        network: turn_context.network.clone(),
         // TODO(zhao-oai): Now that we have ExecExpiration::Cancellation, we
         // should use that instead of an "arbitrarily large" timeout here.
         expiration: USER_SHELL_TIMEOUT_MS.into(),