fix: address load_factor code review findings

- Fix bulk edit: send 0 instead of null/NaN to clear load_factor
- Fix edit modal: explicit NaN check instead of implicit falsy
- Fix create modal: use ?? instead of || for load_factor
- Add load_factor upper limit validation (max 10000)
- Add //go:build unit tag and self-contained intPtrHelper in test
- Add design intent comments on WaitPlan.MaxConcurrency

backend/internal/service/openai_account_scheduler.go

@@ -342,6 +342,7 @@ func (s *defaultOpenAIAccountScheduler) selectBySessionHash(
 	}
 
 	cfg := s.service.schedulingConfig()
+	// WaitPlan.MaxConcurrency 使用 Concurrency（非 EffectiveLoadFactor），因为 WaitPlan 控制的是 Redis 实际并发槽位等待。
 	if s.service.concurrencyService != nil {
 		return &AccountSelectionResult{
 			Account: account,
@@ -703,6 +704,7 @@ func (s *defaultOpenAIAccountScheduler) selectByLoadBalance(
 	}
 
 	cfg := s.service.schedulingConfig()
+	// WaitPlan.MaxConcurrency 使用 Concurrency（非 EffectiveLoadFactor），因为 WaitPlan 控制的是 Redis 实际并发槽位等待。
 	candidate := selectionOrder[0]
 	return &AccountSelectionResult{
 		Account: candidate.account,