Remove the network proxy config wrapper (#31767)

## Why

`NetworkProxyConfig` only wrapped `NetworkProxySettings` in a single
`network` field. That extra level made runtime callers repeat `.network`
everywhere without representing a real boundary.

## What changed

- move the managed-network fields directly onto `NetworkProxyConfig`
- collapse the matching partial-config wrapper
- update runtime callers and tests to use the direct fields
- keep the user-facing permissions/profile TOML layout unchanged

The internal serialized shape now matches the runtime type itself. This
does not change managed-network behavior or the `config.toml` shape.

codex-rs/core/src/config/mod.rs

@@ -3368,7 +3368,7 @@ impl Config {
                     network_proxy,
                 );
             }
-            configured_network_proxy_config.network.enabled = true;
+            configured_network_proxy_config.enabled = true;
         }
         let approval_policy_was_explicit =
             approval_policy_override.is_some() || cfg.approval_policy.is_some();
@@ -4128,7 +4128,7 @@ impl Config {
                         network_proxy,
                     );
                 }
-                configured_network_proxy_config.network.enabled = true;
+                configured_network_proxy_config.enabled = true;
             }
             configured_network_proxy_config
         } else {