fix(msggateway): avoid reusing Clients backing array

[buvidk1234]

🅰 Please add the issue ID after "Fixes #"

Fixes #

Summary

Fix UserMap.DeleteClients to avoid overwriting client slices that were previously returned by GetAll.

Problem

DeleteClients used result.Clients[:0] before rebuilding the remaining client list. This preserves the original capacity, so subsequent append calls reuse and overwrite the same backing array.

Callers such as RPC push or multi-login checks may already hold a slice returned by GetAll and iterate it after UserMap releases its lock. When DeleteClients runs concurrently, those callers can observe mutated slice contents, for example [A, B, C] becoming [B, C, C].

Changes

Use result.Clients[:0:0] so the rebuilt client list gets a new backing array while keeping the existing filtering logic unchanged.