fix(go-models): harden LongCat default transport handling (#15340)

## Summary
- Harden `NewLongCatModel` 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 `TestLongCatNewModelWithCustomDefaultTransport` regression
coverage.

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

internal/entity/models/longcat.go

@@ -60,7 +60,15 @@ type LongCatModel struct {
 // long-lived SSE streams in ChatStreamlyWithSender. Non-streaming
 // callers wrap each request with context.WithTimeout instead.
 func NewLongCatModel(baseURL map[string]string, urlSuffix URLSuffix) *LongCatModel {
-	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/longcat_test.go

@@ -9,6 +9,12 @@ import (
 	"testing"
 )
 
+type roundTripperFunc func(*http.Request) (*http.Response, error)
+
+func (f roundTripperFunc) RoundTrip(r *http.Request) (*http.Response, error) {
+	return f(r)
+}
+
 func newLongCatServer(t *testing.T, expectedPath string, handler func(t *testing.T, r *http.Request, body map[string]interface{}, w http.ResponseWriter)) *httptest.Server {
 	t.Helper()
 	return httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -85,6 +91,20 @@ func TestLongCatName(t *testing.T) {
 	}
 }
 
+func TestLongCatNewModelWithCustomDefaultTransport(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 := NewLongCatModel(map[string]string{"default": "http://unused"}, URLSuffix{}); model == nil {
+		t.Fatal("NewLongCatModel returned nil")
+	}
+}
+
 func TestLongCatChatHappyPath(t *testing.T) {
 	srv := newLongCatServer(t, "/openai/v1/chat/completions", func(t *testing.T, _ *http.Request, body map[string]interface{}, w http.ResponseWriter) {
 		if body["model"] != "LongCat-Flash-Chat" {