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/openai_gateway_service.go

@@ -332,7 +332,7 @@ func (s *OpenAIGatewayService) tryStickySessionHit(ctx context.Context, groupID
 
 	// 检查账号是否需要清理粘性会话
 	// Check if sticky session should be cleared
-	if shouldClearStickySession(account) {
+	if shouldClearStickySession(account, requestedModel) {
 		_ = s.cache.DeleteSessionAccountID(ctx, derefGroupID(groupID), cacheKey)
 		return nil
 	}
@@ -498,7 +498,7 @@ func (s *OpenAIGatewayService) SelectAccountWithLoadAwareness(ctx context.Contex
 		if err == nil && accountID > 0 && !isExcluded(accountID) {
 			account, err := s.getSchedulableAccount(ctx, accountID)
 			if err == nil {
-				clearSticky := shouldClearStickySession(account)
+				clearSticky := shouldClearStickySession(account, requestedModel)
 				if clearSticky {
 					_ = s.cache.DeleteSessionAccountID(ctx, derefGroupID(groupID), "openai:"+sessionHash)
 				}
@@ -1087,30 +1087,6 @@ func (s *OpenAIGatewayService) handleErrorResponse(ctx context.Context, resp *ht
 		)
 	}
 
-	if status, errType, errMsg, matched := applyErrorPassthroughRule(
-		c,
-		PlatformOpenAI,
-		resp.StatusCode,
-		body,
-		http.StatusBadGateway,
-		"upstream_error",
-		"Upstream request failed",
-	); matched {
-		c.JSON(status, gin.H{
-			"error": gin.H{
-				"type":    errType,
-				"message": errMsg,
-			},
-		})
-		if upstreamMsg == "" {
-			upstreamMsg = errMsg
-		}
-		if upstreamMsg == "" {
-			return nil, fmt.Errorf("upstream error: %d (passthrough rule matched)", resp.StatusCode)
-		}
-		return nil, fmt.Errorf("upstream error: %d (passthrough rule matched) message=%s", resp.StatusCode, upstreamMsg)
-	}
-
 	// Check custom error codes
 	if !account.ShouldHandleErrorCode(resp.StatusCode) {
 		appendOpsUpstreamError(c, OpsUpstreamErrorEvent{