fix(go-models): harden N1N default transport handling (#15351)

## Summary
- Harden `NewN1NModel` to avoid panics when `http.DefaultTransport` is a
custom non-`*http.Transport` RoundTripper.
- Fallback to a safe transport (`ProxyFromEnvironment`) while preserving
existing pooling/timeout settings.
- Add `n1n_test.go` with coverage for name/factory plus
`TestN1NNewModelWithCustomDefaultTransport`.


Co-authored-by: Cursor <cursoragent@cursor.com>

internal/entity/models/n1n.go

@@ -54,7 +54,15 @@ type N1NModel struct {
 // long-lived SSE streams in ChatStreamlyWithSender. Non-streaming
 // callers wrap each request with context.WithTimeout instead.
 func NewN1NModel(baseURL map[string]string, urlSuffix URLSuffix) *N1NModel {
-	transport := http.DefaultTransport.(*http.Transport).Clone()
+	defaultTransport, ok := http.DefaultTransport.(*http.Transport)
+	var transport *http.Transport
+	if ok {
+		transport = defaultTransport.Clone()
+	} else {
+		transport = &http.Transport{
+			Proxy: http.ProxyFromEnvironment,
+		}
+	}
 	transport.MaxIdleConns = 100
 	transport.MaxIdleConnsPerHost = 10
 	transport.IdleConnTimeout = 90 * time.Second

internal/entity/models/n1n_test.go

@@ -0,0 +1,49 @@
+package models
+
+import (
+	"net/http"
+	"testing"
+)
+
+type roundTripperFunc func(*http.Request) (*http.Response, error)
+
+func (f roundTripperFunc) RoundTrip(r *http.Request) (*http.Response, error) {
+	return f(r)
+}
+
+func newN1NForTest(baseURL string) *N1NModel {
+	return NewN1NModel(
+		map[string]string{"default": baseURL},
+		URLSuffix{Chat: "chat/completions", Models: "models"},
+	)
+}
+
+func TestN1NName(t *testing.T) {
+	if got := newN1NForTest("http://unused").Name(); got != "n1n" {
+		t.Errorf("Name()=%q, want %q", got, "n1n")
+	}
+}
+
+func TestN1NFactory(t *testing.T) {
+	driver, err := NewModelFactory().CreateModelDriver("n1n", map[string]string{"default": "http://unused"}, URLSuffix{})
+	if err != nil {
+		t.Fatalf("CreateModelDriver: %v", err)
+	}
+	if _, ok := driver.(*N1NModel); !ok {
+		t.Fatalf("driver type=%T, want *N1NModel", driver)
+	}
+}
+
+func TestN1NNewModelWithCustomDefaultTransport(t *testing.T) {
+	original := http.DefaultTransport
+	http.DefaultTransport = roundTripperFunc(func(*http.Request) (*http.Response, error) {
+		return nil, nil
+	})
+	t.Cleanup(func() {
+		http.DefaultTransport = original
+	})
+
+	if model := NewN1NModel(map[string]string{"default": "http://unused"}, URLSuffix{}); model == nil {
+		t.Fatal("NewN1NModel returned nil")
+	}
+}