Add upstream model list fetching and a Codex-specific model section

internal/chatcmpl/client_test.go

@@ -73,6 +73,49 @@ func TestCreateStreamRejectsNon2xx(t *testing.T) {
 	}
 }
 
+func TestListModels(t *testing.T) {
+	var gotPath, gotAuth string
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotPath = r.URL.Path
+		gotAuth = r.Header.Get("Authorization")
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"object":"list","data":[{"id":"deepseek-chat","object":"model","owned_by":"deepseek"},{"id":"deepseek-reasoner"}]}`))
+	}))
+	defer upstream.Close()
+
+	client := NewClient(upstream.URL, "sk-up")
+	list, err := client.ListModels(context.Background())
+	if err != nil {
+		t.Fatalf("ListModels: %v", err)
+	}
+	if gotPath != "/models" {
+		t.Errorf("path = %q, want /models", gotPath)
+	}
+	if gotAuth != "Bearer sk-up" {
+		t.Errorf("authorization = %q", gotAuth)
+	}
+	if len(list.Data) != 2 || list.Data[0].ID != "deepseek-chat" {
+		t.Fatalf("models = %+v", list.Data)
+	}
+}
+
+func TestListModelsUpstreamError(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusUnauthorized)
+		_, _ = w.Write([]byte(`{"error":{"message":"bad key"}}`))
+	}))
+	defer upstream.Close()
+
+	_, err := NewClient(upstream.URL, "sk-bad").ListModels(context.Background())
+	var ue *UpstreamError
+	if !errors.As(err, &ue) {
+		t.Fatalf("error = %v, want *UpstreamError", err)
+	}
+	if ue.StatusCode != http.StatusUnauthorized || ue.Message != "bad key" {
+		t.Errorf("upstream error = %+v", ue)
+	}
+}
+
 func TestStreamNext(t *testing.T) {
 	sse := strings.Join([]string{
 		`data: {"id":"1","choices":[{"index":0,"delta":{"content":"Hel"}}]}`,