fix: resolve CI failures — gofmt, unused functions, test parameter mismatches

- gofmt: user.go, config_test.go, group_handler.go, smart_retry_test.go
- Remove unused: mergeGroupIDs, resolveProxyURL, "time" import
- Fix api_contract_test.go: remove extra Sora args from NewAdminService,
  NewSettingHandler, NewAccountHandler; remove Sora field expectations
- Fix account_test_service_openai_test.go: restore test helpers

backend/internal/service/account_test_service_openai_test.go

@@ -4,6 +4,7 @@ package service
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"net/http"
 	"net/http/httptest"
@@ -13,8 +14,43 @@ import (
 
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/require"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/tlsfingerprint"
 )
 
+// --- shared test helpers ---
+
+type queuedHTTPUpstream struct {
+	responses []*http.Response
+	requests  []*http.Request
+	tlsFlags  []bool
+}
+
+func (u *queuedHTTPUpstream) Do(_ *http.Request, _ string, _ int64, _ int) (*http.Response, error) {
+	return nil, fmt.Errorf("unexpected Do call")
+}
+
+func (u *queuedHTTPUpstream) DoWithTLS(req *http.Request, _ string, _ int64, _ int, profile *tlsfingerprint.Profile) (*http.Response, error) {
+	u.requests = append(u.requests, req)
+	u.tlsFlags = append(u.tlsFlags, profile != nil)
+	if len(u.responses) == 0 {
+		return nil, fmt.Errorf("no mocked response")
+	}
+	resp := u.responses[0]
+	u.responses = u.responses[1:]
+	return resp, nil
+}
+
+func newJSONResponse(status int, body string) *http.Response {
+	return &http.Response{
+		StatusCode: status,
+		Header:     make(http.Header),
+		Body:       io.NopCloser(strings.NewReader(body)),
+	}
+}
+
+// --- test functions ---
+
 func newTestContext() (*gin.Context, *httptest.ResponseRecorder) {
 	gin.SetMode(gin.TestMode)
 	rec := httptest.NewRecorder()