feat(openai): 极致优化 OAuth 链路并补齐性能守护

- 优化 /v1/responses 热路径，减少重复解析与不必要拷贝\n- 优化并发与 token 竞争路径并补齐运行指标\n- 补充 OpenAI/Ops 相关单元测试与回归用例\n- 新增灰度阈值守护与压测脚本，支撑发布验收

backend/internal/handler/gateway_helper_fastpath_test.go

@@ -0,0 +1,114 @@
+package handler
+
+import (
+	"context"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/stretchr/testify/require"
+)
+
+type concurrencyCacheMock struct {
+	acquireUserSlotFn    func(ctx context.Context, userID int64, maxConcurrency int, requestID string) (bool, error)
+	acquireAccountSlotFn func(ctx context.Context, accountID int64, maxConcurrency int, requestID string) (bool, error)
+	releaseUserCalled    int32
+	releaseAccountCalled int32
+}
+
+func (m *concurrencyCacheMock) AcquireAccountSlot(ctx context.Context, accountID int64, maxConcurrency int, requestID string) (bool, error) {
+	if m.acquireAccountSlotFn != nil {
+		return m.acquireAccountSlotFn(ctx, accountID, maxConcurrency, requestID)
+	}
+	return false, nil
+}
+
+func (m *concurrencyCacheMock) ReleaseAccountSlot(ctx context.Context, accountID int64, requestID string) error {
+	atomic.AddInt32(&m.releaseAccountCalled, 1)
+	return nil
+}
+
+func (m *concurrencyCacheMock) GetAccountConcurrency(ctx context.Context, accountID int64) (int, error) {
+	return 0, nil
+}
+
+func (m *concurrencyCacheMock) IncrementAccountWaitCount(ctx context.Context, accountID int64, maxWait int) (bool, error) {
+	return true, nil
+}
+
+func (m *concurrencyCacheMock) DecrementAccountWaitCount(ctx context.Context, accountID int64) error {
+	return nil
+}
+
+func (m *concurrencyCacheMock) GetAccountWaitingCount(ctx context.Context, accountID int64) (int, error) {
+	return 0, nil
+}
+
+func (m *concurrencyCacheMock) AcquireUserSlot(ctx context.Context, userID int64, maxConcurrency int, requestID string) (bool, error) {
+	if m.acquireUserSlotFn != nil {
+		return m.acquireUserSlotFn(ctx, userID, maxConcurrency, requestID)
+	}
+	return false, nil
+}
+
+func (m *concurrencyCacheMock) ReleaseUserSlot(ctx context.Context, userID int64, requestID string) error {
+	atomic.AddInt32(&m.releaseUserCalled, 1)
+	return nil
+}
+
+func (m *concurrencyCacheMock) GetUserConcurrency(ctx context.Context, userID int64) (int, error) {
+	return 0, nil
+}
+
+func (m *concurrencyCacheMock) IncrementWaitCount(ctx context.Context, userID int64, maxWait int) (bool, error) {
+	return true, nil
+}
+
+func (m *concurrencyCacheMock) DecrementWaitCount(ctx context.Context, userID int64) error {
+	return nil
+}
+
+func (m *concurrencyCacheMock) GetAccountsLoadBatch(ctx context.Context, accounts []service.AccountWithConcurrency) (map[int64]*service.AccountLoadInfo, error) {
+	return map[int64]*service.AccountLoadInfo{}, nil
+}
+
+func (m *concurrencyCacheMock) GetUsersLoadBatch(ctx context.Context, users []service.UserWithConcurrency) (map[int64]*service.UserLoadInfo, error) {
+	return map[int64]*service.UserLoadInfo{}, nil
+}
+
+func (m *concurrencyCacheMock) CleanupExpiredAccountSlots(ctx context.Context, accountID int64) error {
+	return nil
+}
+
+func TestConcurrencyHelper_TryAcquireUserSlot(t *testing.T) {
+	cache := &concurrencyCacheMock{
+		acquireUserSlotFn: func(ctx context.Context, userID int64, maxConcurrency int, requestID string) (bool, error) {
+			return true, nil
+		},
+	}
+	helper := NewConcurrencyHelper(service.NewConcurrencyService(cache), SSEPingFormatNone, time.Second)
+
+	release, acquired, err := helper.TryAcquireUserSlot(context.Background(), 101, 2)
+	require.NoError(t, err)
+	require.True(t, acquired)
+	require.NotNil(t, release)
+
+	release()
+	require.Equal(t, int32(1), atomic.LoadInt32(&cache.releaseUserCalled))
+}
+
+func TestConcurrencyHelper_TryAcquireAccountSlot_NotAcquired(t *testing.T) {
+	cache := &concurrencyCacheMock{
+		acquireAccountSlotFn: func(ctx context.Context, accountID int64, maxConcurrency int, requestID string) (bool, error) {
+			return false, nil
+		},
+	}
+	helper := NewConcurrencyHelper(service.NewConcurrencyService(cache), SSEPingFormatNone, time.Second)
+
+	release, acquired, err := helper.TryAcquireAccountSlot(context.Background(), 201, 1)
+	require.NoError(t, err)
+	require.False(t, acquired)
+	require.Nil(t, release)
+	require.Equal(t, int32(0), atomic.LoadInt32(&cache.releaseAccountCalled))
+}