fix: Correct message count display in discussions

Description:

When creating a discussion within a team, channel, or direct message and sending messages there, the number of messages reflected back in the main thread (team, channel, or direct message) is always one less than the actual count.

Steps to Reproduce:

1. Open a team, channel, or direct message from the left pane.
2. Send a message, tap on it, click on the three-dot menu, and select "Create Discussion".
3. Send multiple messages within the discussion.
4. Navigate back to the original team, channel, or direct message.
5. Observe that the number of messages displayed is always one less than the actual count.

Expected Behavior:

• The main thread should correctly reflect the total number of messages sent in the discussion.

Actual Behavior:

https://github.com/user-attachments/assets/7e4481cc-c243-4cb3-97ea-470643bfed34

Server/Client Information:

Server:

• Operating System: Windows 11 (using WSL Terminal)
• Rocket.Chat Server Version: 7.5.0-develop
• Node.js Version: 22.13.1
• Yarn Version: 4.6.0

Client:

• Client Type: Browser (Microsoft Edge)
• Access URL: http://localhost:3000

Additional Information:

• WSL Terminal: Used for installing dependencies.

Are you working on this?

Yes, I am working on this.

Reproduce the issue by creating a discussion from a team, channel, or direct message, sending multiple messages, and checking the count in the original thread. Trace the discussion message-count display from that flow and verify that the displayed total matches the actual number of messages in all three contexts.