fix: resolve cherry-pick compilation and test issues

- Add int64(0) param to SelectAccountWithLoadAwareness callers (signature change from channel scheduling refactor)
- Add UsageMapHook type and struct field to StreamingProcessor
- Revert Claude Max cache billing code to upstream/main (not part of channel feature)
- Revert credits overages logic to upstream/main (non-channel change)
- Remove Instructions field reference (non-channel OpenAI feature)
- Restore sora_client_handler_test.go from upstream + add channel service nil params

backend/internal/service/ops_retry.go

@@ -519,7 +519,7 @@ func (s *OpsService) selectAccountForRetry(ctx context.Context, reqType opsRetry
 		if s.gatewayService == nil {
 			return nil, fmt.Errorf("gateway service not available")
 		}
-		return s.gatewayService.SelectAccountWithLoadAwareness(ctx, groupID, "", model, excludedIDs, "") // 重试不使用会话限制
+		return s.gatewayService.SelectAccountWithLoadAwareness(ctx, groupID, "", model, excludedIDs, "", int64(0)) // 重试不使用会话限制
 	default:
 		return nil, fmt.Errorf("unsupported retry type: %s", reqType)
 	}