fix(lint): resolve remaining nil dereference warnings

backend/internal/service/openai_images_incomplete_test.go

@@ -36,9 +36,7 @@ func TestExtractImagesUpstreamError_IncompleteIsRetryable(t *testing.T) {
 func TestExtractImagesUpstreamError_IncompleteContentFilterNotRetryable(t *testing.T) {
 	body := "data: {\"type\":\"response.incomplete\",\"response\":{\"id\":\"r\",\"status\":\"incomplete\",\"incomplete_details\":{\"reason\":\"content_filter\"}}}\n\n"
 	got := extractOpenAIImagesUpstreamError([]byte(body))
-	if got == nil {
-		t.Fatal("content_filter incomplete should produce error")
-	}
+	require.NotNil(t, got)
 	if got.StatusCode != http.StatusBadRequest {
 		t.Fatalf("content_filter should be 400 (non-retryable), got %d", got.StatusCode)
 	}

backend/internal/service/ops_service_user_error_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
+	"github.com/stretchr/testify/require"
 )
 
 type stubOpsRepoForUserErr struct {
@@ -119,9 +120,7 @@ func TestGetUserErrorRequestDetail_OwnershipEnforced(t *testing.T) {
 	if err2 != nil {
 		t.Fatalf("expected no error for legitimate access, got %v", err2)
 	}
-	if got2 == nil {
-		t.Fatal("expected non-nil detail for legitimate access")
-	}
+	require.NotNil(t, got2)
 	if got2.ID != 42 {
 		t.Errorf("want ID=42, got %d", got2.ID)
 	}

backend/internal/service/ops_user_error_test.go

@@ -5,6 +5,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/stretchr/testify/require"
 )
 
 func TestMapUserErrorCategory(t *testing.T) {
@@ -132,9 +134,7 @@ func TestToUserErrorRequestDetail_WhitelistAndRedacts(t *testing.T) {
 	}
 
 	out := ToUserErrorRequestDetail(src)
-	if out == nil {
-		t.Fatal("expected non-nil detail")
-	}
+	require.NotNil(t, out)
 
 	// 基础字段正确映射
 	if out.ID != 999 {