fix(go-models): harden ModelScope default transport handling (#15339)

## Summary
- Harden `NewModelScopeModel` 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 `TestModelScopeNewModelWithCustomDefaultTransport` regression
coverage.

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Jin Hai <haijin.chn@gmail.com>

internal/entity/models/modelscope.go

@@ -72,7 +72,15 @@ type modelscopeModelListResponse struct {
 
 // NewModelScopeModel creates a new ModelScope model instance.
 func NewModelScopeModel(baseURL map[string]string, urlSuffix URLSuffix) *ModelScopeModel {
-	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/modelscope_test.go

@@ -26,6 +26,12 @@ import (
 	"time"
 )
 
+type roundTripperFunc func(*http.Request) (*http.Response, error)
+
+func (f roundTripperFunc) RoundTrip(r *http.Request) (*http.Response, error) {
+	return f(r)
+}
+
 func newModelScopeForTest(baseURL string) *ModelScopeModel {
 	return NewModelScopeModel(
 		map[string]string{"default": baseURL},
@@ -79,6 +85,20 @@ func TestModelScopeFactoryRoute(t *testing.T) {
 	}
 }
 
+func TestModelScopeNewModelWithCustomDefaultTransport(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 := NewModelScopeModel(map[string]string{"default": "http://unused"}, URLSuffix{}); model == nil {
+		t.Fatal("NewModelScopeModel returned nil")
+	}
+}
+
 func TestModelScopeChatHappyPathNormalizesBaseURLAndOmitsEmptyAuth(t *testing.T) {
 	var seen map[string]interface{}
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {