Discussions message count misleading

Description:

The message count for discussions in the parent channel is misleading. It counts system messages, even if "Hide System Messages" is enabled. It also counts deleted messages. Further, it does not pluralize.

Seems like the counting is only triggered by new (non-system) messages.

Steps to reproduce:

1. Go to any channel.
2. Create a discussion, enable "Hide System Messages".
3. Join the discussion with a different user. Notice: The message count in the parent channel does not change.
4. Write a message with the user from the last step. The message count increases by 2.
5. Delete the message. The message count does not decrease.

Expected behavior:

System messages should not be counted, independent of the state of "Hide System Messages". Deleted messages should reduce the message count. Last message time should be refreshed accordingly (right next to the X messages button).

Actual behavior:

System messages are counted, but only after a regular message is written. Deleted messages do not decrease the message count.

Server Setup Information:

• Version of Rocket.Chat Server: 6.5.3
• Operating System: Ubuntu 22.04 LTS
• Deployment Method: Docker
• Number of Running Instances:
• DB Replicaset Oplog:
• NodeJS Version:
• MongoDB Version:

Client Setup Information

• Desktop App or Browser Version: Firefox 122.0
• Operating System: Ubuntu 23.10

Additional context

Relevant logs:

Reproduce the discussion flow described in the issue using Rocket.Chat 6.5.3; the payload names no source files, tests, or entry points, so those must be located first. Done means system and deleted messages are excluded from the parent-channel count, the count updates after joins, posts, and deletions, pluralization is correct, and the last-message time refreshes.