[codex] compact network context rendering (#21875)

## Why

The model-visible `<network>` context currently repeats indentation and
a pair of XML tags for every allowed or denied domain. Large domain sets
spend a surprising amount of prompt budget on that scaffolding instead
of the actual policy values.

## What changed

- Render allowed domains as one comma-separated `<allowed>` value
instead of one element per domain.
- Render denied domains the same way.
- Keep the full allow/deny domain sets model-visible while updating the
serialization and settings-update coverage for the denser shape.

## Example

Before:
```xml
<network enabled="true">
  <allowed>api.example.test</allowed>
  <allowed>cdn.example.test</allowed>
  <denied>blocked.example.test</denied>
</network>
```

After:
```xml
<network enabled="true"><allowed>api.example.test,cdn.example.test</allowed><denied>blocked.example.test</denied></network>
```

## Validation

- `cargo test -p codex-core environment_context`
- `cargo test -p codex-core
build_settings_update_items_emits_environment_item_for_network_changes`
- Ran a local `codex` session with a real network context containing 121
allowed domains and 42 denied domains, then inspected the raw prompt
with `raw_token_viewer_cli.py`. With the same domain set, the rendered
`<network>` section shrank from 7,175 characters across 161 lines to
3,666 characters on one line, and the containing environment-context
block fell from 6,428 tokens to 5,379 tokens.

codex-rs/core/src/context/environment_context.rs

@@ -96,6 +96,24 @@ impl NetworkContext {
             denied_domains,
         }
     }
+
+    fn render(&self) -> String {
+        let mut rendered = "<network enabled=\"true\">".to_string();
+        Self::push_rendered_domain_element(&mut rendered, "allowed", &self.allowed_domains);
+        Self::push_rendered_domain_element(&mut rendered, "denied", &self.denied_domains);
+        rendered.push_str("</network>");
+        rendered
+    }
+
+    fn push_rendered_domain_element(rendered_network: &mut String, name: &str, domains: &[String]) {
+        if domains.is_empty() {
+            return;
+        }
+
+        rendered_network.push_str(&format!("<{name}>"));
+        rendered_network.push_str(&domains.join(","));
+        rendered_network.push_str(&format!("</{name}>"));
+    }
 }
 
 impl EnvironmentContext {
@@ -288,14 +306,7 @@ impl ContextualUserFragment for EnvironmentContext {
         }
         match &self.network {
             Some(network) => {
-                lines.push("  <network enabled=\"true\">".to_string());
-                for allowed in &network.allowed_domains {
-                    lines.push(format!("    <allowed>{allowed}</allowed>"));
-                }
-                for denied in &network.denied_domains {
-                    lines.push(format!("    <denied>{denied}</denied>"));
-                }
-                lines.push("  </network>".to_string());
+                lines.push(format!("  {}", network.render()));
             }
             None => {
                 // TODO(mbolin): Include this line if it helps the model.

codex-rs/core/src/context/environment_context_tests.rs

@@ -71,11 +71,7 @@ fn serialize_environment_context_with_network() {
   <shell>bash</shell>
   <current_date>2026-02-26</current_date>
   <timezone>America/Los_Angeles</timezone>
-  <network enabled="true">
-    <allowed>api.example.com</allowed>
-    <allowed>*.openai.com</allowed>
-    <denied>blocked.example.com</denied>
-  </network>
+  <network enabled="true"><allowed>api.example.com,*.openai.com</allowed><denied>blocked.example.com</denied></network>
 </environment_context>"#,
         test_path_buf("/repo").display()
     );

codex-rs/core/src/session/tests.rs

@@ -5896,9 +5896,9 @@ async fn build_settings_update_items_emits_environment_item_for_network_changes(
         .into_iter()
         .find(|text| text.contains("<environment_context>"))
         .expect("environment update item should be emitted");
-    assert!(environment_update.contains("<network enabled=\"true\">"));
-    assert!(environment_update.contains("<allowed>api.example.com</allowed>"));
-    assert!(environment_update.contains("<denied>blocked.example.com</denied>"));
+    assert!(environment_update.contains(
+        "<network enabled=\"true\"><allowed>api.example.com</allowed><denied>blocked.example.com</denied></network>"
+    ));
 }
 
 #[tokio::test]