Message search scrolling lags when user has many unreads

Description:

The MessageSearch component suffers from performance issues during scrolling, especially for users with a large number of unread messages or threads.

Tested with const mockBigArray = Array.from({ length: 10000 }, (_, i) => fake-id-${i});
const unread = mockBigArray.includes(message._id); mock a huge number of unreads in MessageSearch.tsx

https://github.com/user-attachments/assets/0507fe14-ee04-4fec-8306-beb63d395efc

Steps to reproduce:

1. Open a room where the user has a high number of unread messages or active threads.
2. Open the Search Messages sidebar.
3. Search for a common term to generate a long list of results.
4. Scroll quickly through the results.

Expected behavior:

Scrolling should be smooth regardless of the number of unread messages or threads the user has.

Actual behavior:

Scrolling is "heavy" and stuttery. React Profiler shows high render durations for list items because each item is waiting for multiple linear array scans to complete.

Server Setup Information:

• Version of Rocket.Chat Server: 8.3.0-develop
• Operating System: Fedora Workstation 41
• Deployment Method: Local development
• NodeJS Version: 22.16.0

Client Setup Information

• Desktop App or Browser Version: Brave
• Operating System: Fedora Workstation 41

Start in MessageSearch.tsx and reproduce the issue with a large unread-message array while profiling rapid scrolling through search results. Trace the list-item render path and the reported linear array scans; done means scrolling remains smooth with many unreads or threads and the observed render durations are reduced.