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/app-server/src/message_processor.rs

@@ -158,6 +158,12 @@ impl MessageProcessor {
             ClientRequest::ConfigBatchWrite { request_id, params } => {
                 self.handle_config_batch_write(request_id, params).await;
             }
+            ClientRequest::ConfigRequirementsRead {
+                request_id,
+                params: _,
+            } => {
+                self.handle_config_requirements_read(request_id).await;
+            }
             other => {
                 self.codex_message_processor.process_request(other).await;
             }
@@ -210,4 +216,11 @@ impl MessageProcessor {
             Err(error) => self.outgoing.send_error(request_id, error).await,
         }
     }
+
+    async fn handle_config_requirements_read(&self, request_id: RequestId) {
+        match self.config_api.config_requirements_read().await {
+            Ok(response) => self.outgoing.send_response(request_id, response).await,
+            Err(error) => self.outgoing.send_error(request_id, error).await,
+        }
+    }
 }