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

internal/gateway/gateway_test.go

@@ -23,6 +23,11 @@ type fakeUpstream struct {
 }
 
 func (f *fakeUpstream) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if r.Method == http.MethodGet && r.URL.Path == "/models" {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = io.WriteString(w, `{"object":"list","data":[{"id":"ds-chat"},{"id":"ds-haiku"}]}`)
+		return
+	}
 	if r.URL.Path != "/chat/completions" {
 		w.WriteHeader(http.StatusNotFound)
 		return
@@ -683,6 +688,61 @@ func TestApplyRejectsWrongTarget(t *testing.T) {
 	}
 }
 
+func TestFetchModels(t *testing.T) {
+	env := newTestEnv(t)
+	resp := env.post(t, "/api/fetch-models", map[string]any{
+		"base_url": env.upstreamURL,
+		"api_key":  "up-key",
+	}, "")
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("status = %d", resp.StatusCode)
+	}
+	var out struct {
+		Items []struct {
+			ID string `json:"id"`
+		} `json:"items"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(out.Items) != 2 || out.Items[0].ID != "ds-chat" {
+		t.Errorf("items = %+v", out.Items)
+	}
+}
+
+func TestFetchModelsRejectsBadInput(t *testing.T) {
+	env := newTestEnv(t)
+	for _, body := range []map[string]any{
+		{"base_url": env.upstreamURL},
+		{"api_key": "up-key"},
+		{"base_url": "ftp://x", "api_key": "k"},
+	} {
+		resp := env.post(t, "/api/fetch-models", body, "")
+		resp.Body.Close()
+		if resp.StatusCode != http.StatusBadRequest {
+			t.Errorf("body %v status = %d, want 400", body, resp.StatusCode)
+		}
+	}
+}
+
+func TestFetchModelsUpstreamFailure(t *testing.T) {
+	env := newTestEnv(t)
+	broken := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusUnauthorized)
+		_, _ = io.WriteString(w, `{"error":{"message":"bad key"}}`)
+	}))
+	defer broken.Close()
+	resp := env.post(t, "/api/fetch-models", map[string]any{
+		"base_url": broken.URL,
+		"api_key":  "sk-bad",
+	}, "")
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusBadGateway {
+		t.Errorf("status = %d, want 502", resp.StatusCode)
+	}
+}
+
 func TestListProvidersFiltersByTarget(t *testing.T) {
 	env := newCodexTestEnv(t)
 	codexItems := env.get(t, "/api/providers?target=codex")["items"].([]any)