Add models endpoint (#7603)

- Use the codex-api crate to introduce models endpoint. 
- Add `models` to codex core tests helpers
- Add `ModelsInfo` for the endpoint return type

codex-rs/core/tests/common/responses.rs

@@ -3,6 +3,7 @@ use std::sync::Mutex;
 
 use anyhow::Result;
 use base64::Engine;
+use codex_protocol::openai_models::ModelsResponse;
 use serde_json::Value;
 use wiremock::BodyPrintLimit;
 use wiremock::Match;
@@ -193,6 +194,38 @@ impl ResponsesRequest {
     }
 }
 
+#[derive(Debug, Clone)]
+pub struct ModelsMock {
+    requests: Arc<Mutex<Vec<wiremock::Request>>>,
+}
+
+impl ModelsMock {
+    fn new() -> Self {
+        Self {
+            requests: Arc::new(Mutex::new(Vec::new())),
+        }
+    }
+
+    pub fn requests(&self) -> Vec<wiremock::Request> {
+        self.requests.lock().unwrap().clone()
+    }
+
+    pub fn single_request_path(&self) -> String {
+        let requests = self.requests.lock().unwrap();
+        if requests.len() != 1 {
+            panic!("expected 1 request, got {}", requests.len());
+        }
+        requests.first().unwrap().url.path().to_string()
+    }
+}
+
+impl Match for ModelsMock {
+    fn matches(&self, request: &wiremock::Request) -> bool {
+        self.requests.lock().unwrap().push(request.clone());
+        true
+    }
+}
+
 impl Match for ResponseMock {
     fn matches(&self, request: &wiremock::Request) -> bool {
         self.requests
@@ -560,6 +593,14 @@ fn compact_mock() -> (MockBuilder, ResponseMock) {
     (mock, response_mock)
 }
 
+fn models_mock() -> (MockBuilder, ModelsMock) {
+    let models_mock = ModelsMock::new();
+    let mock = Mock::given(method("GET"))
+        .and(path_regex(".*/models$"))
+        .and(models_mock.clone());
+    (mock, models_mock)
+}
+
 pub async fn mount_sse_once_match<M>(server: &MockServer, matcher: M, body: String) -> ResponseMock
 where
     M: wiremock::Match + Send + Sync + 'static,
@@ -616,11 +657,29 @@ pub async fn mount_compact_json_once(server: &MockServer, body: serde_json::Valu
     response_mock
 }
 
+pub async fn mount_models_once(server: &MockServer, body: ModelsResponse) -> ModelsMock {
+    let (mock, models_mock) = models_mock();
+    mock.respond_with(
+        ResponseTemplate::new(200)
+            .insert_header("content-type", "application/json")
+            .set_body_json(body.clone()),
+    )
+    .up_to_n_times(1)
+    .mount(server)
+    .await;
+    models_mock
+}
+
 pub async fn start_mock_server() -> MockServer {
-    MockServer::builder()
+    let server = MockServer::builder()
         .body_print_limit(BodyPrintLimit::Limited(80_000))
         .start()
-        .await
+        .await;
+
+    // Provide a default `/models` response so tests remain hermetic when the client queries it.
+    let _ = mount_models_once(&server, ModelsResponse { models: Vec::new() }).await;
+
+    server
 }
 
 #[derive(Clone)]