CombineHub & SignalRHub Flaw: Malicious Clients Can Falsely Acknowledge Any User's Messages

A critical security vulnerability in the CombineHub and SignalRHub components allows any connected client to falsely acknowledge and suppress messages intended for other users. The flaw stems from the `AcknowledgeMessage` method, which accepts only a `requestId` without validating the caller's identity against the intended recipient. Since `requestId` values are broadcast to all clients via `Clients.All`, a malicious or compromised client can use any known ID to mark another user's messages as acknowledged, halting critical retry mechanisms.

The issue, flagged as a major potential risk by an automated review, centers on the lack of authorization in the acknowledgment process. The current implementation in `@Backend/Helper/CombineHub.cs` (lines 6-22) does not store or check a mapping between a `requestId` and its intended `userId`. This means the system trusts the caller's claim without verifying they are the legitimate recipient, relying solely on the `Context.UserIdentifier` for identity without cross-referencing it against the message's target.

This design oversight creates a direct pathway for message suppression and system integrity failure. A fix requires modifying `AcknowledgeMessage` to either accept the intended `userId` as a parameter or, more securely, to infer it from a tracker that stores `requestId → userId` mappings. Validation must then occur against `Context.UserIdentifier` before any acknowledgment is processed. Without this change, the reliability of the entire message retry system is compromised, exposing dependent workflows to potential disruption by a single bad actor.