fix(lint): use require.NotNil for staticcheck SA5011

Replace if-then-t.Fatal nil checks with testify require.NotNil so
staticcheck can prove the pointer is non-nil before field access.
Fixes SA5011 in payment_handler_test, parse_test, and
openai_images_incomplete_test.

backend/internal/handler/admin/payment_handler_test.go

@@ -8,6 +8,7 @@ import (
 
 	dbent "github.com/Wei-Shaw/sub2api/ent"
 	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/stretchr/testify/require"
 )
 
 func TestSanitizeAdminPaymentOrderForResponseAddsCurrency(t *testing.T) {
@@ -32,9 +33,7 @@ func TestSanitizeAdminPaymentOrderForResponseAddsCurrency(t *testing.T) {
 	}
 
 	got := sanitizeAdminPaymentOrderForResponse(order)
-	if got == nil {
-		t.Fatal("expected sanitized order")
-	}
+	require.NotNil(t, got)
 	if got.Currency != "USD" {
 		t.Fatalf("expected currency USD, got %q", got.Currency)
 	}

backend/internal/pkg/proxyurl/parse_test.go

@@ -3,6 +3,8 @@ package proxyurl
 import (
 	"strings"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 func TestParse_空字符串直连(t *testing.T) {
@@ -39,9 +41,7 @@ func TestParse_有效HTTP代理(t *testing.T) {
 	if trimmed != "http://proxy.example.com:8080" {
 		t.Errorf("trimmed 不匹配: got %q", trimmed)
 	}
-	if parsed == nil {
-		t.Fatal("parsed 不应为 nil")
-	}
+	require.NotNil(t, parsed)
 	if parsed.Host != "proxy.example.com:8080" {
 		t.Errorf("Host 不匹配: got %q", parsed.Host)
 	}

backend/internal/service/openai_images_incomplete_test.go

@@ -9,6 +9,7 @@ import (
 	"testing"
 
 	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
 )
 
 // response.incomplete（生成超时/截断）应被识别为可重试的 502 上游错误，触发 failover。
@@ -16,9 +17,7 @@ func TestExtractImagesUpstreamError_IncompleteIsRetryable(t *testing.T) {
 	body := "data: {\"type\":\"response.created\",\"response\":{\"id\":\"resp_1\"}}\n\n" +
 		"data: {\"type\":\"response.incomplete\",\"response\":{\"id\":\"resp_1\",\"status\":\"incomplete\",\"incomplete_details\":{\"reason\":\"max_output_tokens\"}}}\n\n"
 	got := extractOpenAIImagesUpstreamError([]byte(body))
-	if got == nil {
-		t.Fatal("incomplete event should produce an upstream error, got nil")
-	}
+	require.NotNil(t, got)
 	if got.StatusCode != http.StatusBadGateway {
 		t.Fatalf("incomplete(max_output_tokens) should be 502 retryable, got %d", got.StatusCode)
 	}