GH-1328 Fix name update handling.

[vLuckyyy]

Already tested on production.

[gemini-code-assist]

Code Review

This pull request refactors the UserManager to ensure correct handling of user name updates. The updateNameIfChanged method now returns an updated User object (a new instance if the name changed) and correctly manages the usersByName map by removing old name mappings, which improves consistency across cached and persisted user data. However, the current implementation of updateNameIfChanged introduces a critical race condition that can lead to data corruption if multiple threads concurrently update the same user's name. It is recommended to implement a synchronized block or a per-user locking mechanism to ensure thread safety.