fix(msggateway): avoid closed channel panic in super group push

[buvidk1234]

🅰 Please add the issue ID after "Fixes #"

Fixes #

Summary

Fix a panic risk in SuperGroupOnlineBatchPushOneMsg when queue enqueue fails.

Problem

SuperGroupOnlineBatchPushOneMsg uses a result channel to collect per-user push results and closes the channel when all pending users are handled.

In the PushCtx error branch, the previous code decremented the pending counter and could close the result channel before sending the fallback result. If the failed enqueue was the last pending item, the following send would panic:

panic: send on closed channel

This can happen on abnormal paths such as request context cancellation while the in-memory queue is unable to accept another task, or when the queue has already stopped.

Changes

• Centralized result sending and completion accounting in pushResult.
• Ensured both success and PushCtx error paths send the user result before decrementing the pending counter.
• Kept the last completed sender responsible for closing the result channel.