feat: include [experimental_network] in <environment_context> (#11044)

If `NetworkConstraints` is set, then include the relevant settings on `<environment_context>`. Example:

```xml
<environment_context>
  <cwd>/repo</cwd>
  <shell>bash</shell>
  <network enabled="true">
    <allowed>api.example.com</allowed>
    <allowed>*.openai.com</allowed>
    <denied>blocked.example.com</denied>
  </network>
</environment_context>
```

codex-rs/core/src/codex.rs

@@ -2204,9 +2204,9 @@ impl Session {
                 .into(),
             );
         }
-        items.push(ResponseItem::from(EnvironmentContext::new(
-            Some(turn_context.cwd.clone()),
-            shell.as_ref().clone(),
+        items.push(ResponseItem::from(EnvironmentContext::from_turn_context(
+            turn_context,
+            shell.as_ref(),
         )));
         items
     }