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/service/error_passthrough_runtime.go

@@ -1,67 +0,0 @@
-package service
-
-import "github.com/gin-gonic/gin"
-
-const errorPassthroughServiceContextKey = "error_passthrough_service"
-
-// BindErrorPassthroughService 将错误透传服务绑定到请求上下文，供 service 层在非 failover 场景下复用规则。
-func BindErrorPassthroughService(c *gin.Context, svc *ErrorPassthroughService) {
-	if c == nil || svc == nil {
-		return
-	}
-	c.Set(errorPassthroughServiceContextKey, svc)
-}
-
-func getBoundErrorPassthroughService(c *gin.Context) *ErrorPassthroughService {
-	if c == nil {
-		return nil
-	}
-	v, ok := c.Get(errorPassthroughServiceContextKey)
-	if !ok {
-		return nil
-	}
-	svc, ok := v.(*ErrorPassthroughService)
-	if !ok {
-		return nil
-	}
-	return svc
-}
-
-// applyErrorPassthroughRule 按规则改写错误响应；未命中时返回默认响应参数。
-func applyErrorPassthroughRule(
-	c *gin.Context,
-	platform string,
-	upstreamStatus int,
-	responseBody []byte,
-	defaultStatus int,
-	defaultErrType string,
-	defaultErrMsg string,
-) (status int, errType string, errMsg string, matched bool) {
-	status = defaultStatus
-	errType = defaultErrType
-	errMsg = defaultErrMsg
-
-	svc := getBoundErrorPassthroughService(c)
-	if svc == nil {
-		return status, errType, errMsg, false
-	}
-
-	rule := svc.MatchRule(platform, upstreamStatus, responseBody)
-	if rule == nil {
-		return status, errType, errMsg, false
-	}
-
-	status = upstreamStatus
-	if !rule.PassthroughCode && rule.ResponseCode != nil {
-		status = *rule.ResponseCode
-	}
-
-	errMsg = ExtractUpstreamErrorMessage(responseBody)
-	if !rule.PassthroughBody && rule.CustomMessage != nil {
-		errMsg = *rule.CustomMessage
-	}
-
-	// 与现有 failover 场景保持一致：命中规则时统一返回 upstream_error。
-	errType = "upstream_error"
-	return status, errType, errMsg, true
-}