feat(antigravity): comprehensive enhancements - model mapping, rate limiting, scheduling & ops

Key changes:
- Upgrade model mapping: Opus 4.5 → Opus 4.6-thinking with precise matching
- Unified rate limiting: scope-level → model-level with Redis snapshot sync
- Load-balanced scheduling by call count with smart retry mechanism
- Force cache billing support
- Model identity injection in prompts with leak prevention
- Thinking mode auto-handling (max_tokens/budget_tokens fix)
- Frontend: whitelist mode toggle, model mapping validation, status indicators
- Gemini session fallback with Redis Trie O(L) matching
- Ops: enhanced concurrency monitoring, account availability, retry logic
- Migration scripts: 049-051 for model mapping unification

backend/internal/handler/openai_gateway_handler.go

@@ -149,11 +149,6 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 	// Track if we've started streaming (for error handling)
 	streamStarted := false
 
-	// 绑定错误透传服务，允许 service 层在非 failover 错误场景复用规则。
-	if h.errorPassthroughService != nil {
-		service.BindErrorPassthroughService(c, h.errorPassthroughService)
-	}
-
 	// Get subscription info (may be nil)
 	subscription, _ := middleware2.GetSubscriptionFromContext(c)