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-protocol/src/protocol/common.rs

@@ -197,6 +197,11 @@ client_request_definitions! {
         response: v2::ConfigWriteResponse,
     },
 
+    ConfigRequirementsRead => "configRequirements/read" {
+        params: #[ts(type = "undefined")] #[serde(skip_serializing_if = "Option::is_none")] Option<()>,
+        response: v2::ConfigRequirementsReadResponse,
+    },
+
     GetAccount => "account/read" {
         params: v2::GetAccountParams,
         response: v2::GetAccountResponse,
@@ -711,6 +716,22 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn serialize_config_requirements_read() -> Result<()> {
+        let request = ClientRequest::ConfigRequirementsRead {
+            request_id: RequestId::Integer(1),
+            params: None,
+        };
+        assert_eq!(
+            json!({
+                "method": "configRequirements/read",
+                "id": 1,
+            }),
+            serde_json::to_value(&request)?,
+        );
+        Ok(())
+    }
+
     #[test]
     fn serialize_account_login_api_key() -> Result<()> {
         let request = ClientRequest::LoginAccount {