Feat: appServer.requirementList for requirement.toml (#8800)

### Summary
We are exposing requirements via `requirement/list` method from
app-server so that we can conditionally disable the agent mode dropdown
selection in VSCE and correctly setting the default value.

### Sample output
#### `etc/codex/requirements.toml`
<img width="497" height="49" alt="Screenshot 2026-01-06 at 11 32 06 PM"
src="https://github.com/user-attachments/assets/fbd9402e-515f-4b9e-a158-2abb23e866a0"
/>

#### App server response
<img width="1107" height="79" alt="Screenshot 2026-01-06 at 11 30 18 PM"
src="https://github.com/user-attachments/assets/c0d669cd-54ef-4789-a26c-adb2c41950af"
/>

codex-rs/core/src/exec_policy.rs

@@ -421,6 +421,7 @@ mod tests {
     use crate::config_loader::ConfigLayerEntry;
     use crate::config_loader::ConfigLayerStack;
     use crate::config_loader::ConfigRequirements;
+    use crate::config_loader::ConfigRequirementsToml;
     use crate::features::Feature;
     use crate::features::Features;
     use codex_app_server_protocol::ConfigLayerSource;
@@ -441,7 +442,12 @@ mod tests {
             ConfigLayerSource::Project { dot_codex_folder },
             TomlValue::Table(Default::default()),
         );
-        ConfigLayerStack::new(vec![layer], ConfigRequirements::default()).expect("ConfigLayerStack")
+        ConfigLayerStack::new(
+            vec![layer],
+            ConfigRequirements::default(),
+            ConfigRequirementsToml::default(),
+        )
+        .expect("ConfigLayerStack")
     }
 
     #[tokio::test]
@@ -573,7 +579,11 @@ mod tests {
                 TomlValue::Table(Default::default()),
             ),
         ];
-        let config_stack = ConfigLayerStack::new(layers, ConfigRequirements::default())?;
+        let config_stack = ConfigLayerStack::new(
+            layers,
+            ConfigRequirements::default(),
+            ConfigRequirementsToml::default(),
+        )?;
 
         let policy = load_exec_policy(&config_stack).await?;