Message search `from:` and `mention:` filters match unintended usernames when username contains a dot

Description:

When searching messages using the from: or mention: filters with a username that contains a dot (e.g. rocket.cat), the search returns messages from unintended users. This is because the username is inserted directly into a MongoDB $regex pattern without escaping, causing . to act as a regex wildcard ("any character") instead of a literal dot.

This affects any workspace using the default username policy, which allows dots in usernames ([0-9a-zA-Z-_.]+), including the built-in rocket.cat bot.

Steps to reproduce:

1. Ensure you have two users whose usernames differ only at a dot position — e.g. rocket.cat and rocketXcat (or john.doe and johnXdoe).
2. Have both users send messages in a channel you have access to.
3. Open the message search and enter from:rocket.cat.
4. Observe the results.

Expected behavior:

Only messages sent by the user with the exact username rocket.cat should appear in the results.

Actual behavior:

Messages from other users whose usernames match the unescaped pattern such as rocketXcat, rocket_cat, or any username where the dot position is any character are also returned.

Server Setup Information:

• Version of Rocket.Chat Server: 8.3.0-rc.2 (reproduced on develop branch)
• License Type:
• Number of Users: Any (requires at least two users with usernames differing at a dot position)
• Operating System:
• Deployment Method:
• Number of Running Instances:
• DB Replicaset Oplog:
• NodeJS Version:
• MongoDB Version:

Client Setup Information

• Desktop App or Browser Version:
• Operating System:

Additional context

Relevant logs:

No server or browser logs required because the issue is observable purely in search results.

Start at the message-search handling for the from: and mention: filters and locate where usernames are inserted into the MongoDB $regex pattern. Make matching literal for usernames containing dots, then reproduce the rocket.cat versus rocketXcat case and verify that only the exact username's messages are returned.