Unable to tag usernames with accents in them

### Current Behavior

A user of my forum has the username Xhéres . I'm unable to tag him in posts, likely because his username has an accent in it.

### Steps to Reproduce

1. Register a user with the username Xhéres
2. Try to tag the user in a post. The username is autocomplete correctly:


3. After posting, the user is not correctly tagged. Xhéres shows up as not tagged, while all of the others users are successfully tagged:

### Expected Behavior

Tagging users with accents in their username works.

### Screenshots

_No response_

### Environment

- Flarum version: 1.8.9
- Website URL: https://forum.mylittlewordland.com/d/4949-every-ttmik-lesson-levels-1-10
- Webserver: Apache
- Hosting environment: VPS
- PHP version: 8.2.13
- Browser: Firefox 136

### Output of `php flarum info`

```
Flarum core: 1.8.9
PHP version: 8.2.13
MySQL version: 10.3.39-MariaDB
Loaded extensions: [hidden]
+------------------------------+---------+--------+
| Flarum Extensions | | |
+------------------------------+---------+--------+
| ID | Version | Commit |
+------------------------------+---------+--------+
| flarum-tags | v1.8.3 | |
| flarum-suspend | v1.8.4 | |
| flarum-mentions | v1.8.5 | |
| flarum-subscriptions | v1.8.1 | |
| flarum-markdown | v1.8.1 | |
| tpokorra-post-notification | 0.4.8 | |
| noriods-auto-more | v1.0.0 | |
| fof-user-bio | 1.4.2 | |
| fof-upload | 1.7.0 | |
| fof-split | 1.1.1 | |
| fof-sitemap | 2.2.1 | |
| fof-profile-image-crop | 1.1.6 | |
| fof-pretty-mail | 1.1.2 | |
| fof-polls | 2.2.9 | |
| fof-nightmode | 1.6.0 | |
| fof-merge-discussions | 1.4.1 | |
| fof-follow-tags | 1.2.7 | |
| fof-drafts | 1.2.12 | |
| fof-default-user-preferences | 1.2.1 | |
| flarum-sticky | v1.8.2 | |
| flarum-statistics | v1.8.1 | |
| flarum-nicknames | v1.8.2 | |
| flarum-lock | v1.8.2 | |
| flarum-likes | v1.8.1 | |
| flarum-lang-english | v1.8.0 | |
| flarum-flags | v1.8.2 | |
| flarum-emoji | v1.8.1 | |
| blomstra-mark-unread | 0.2.0 | |
| askvortsov-rich-text | v2.1.7 | |
| askvortsov-markdown-tables | v1.2.1 | |
| askvortsov-categories | v3.1.0 | |
+------------------------------+---------+--------+
Base URL: https://forum.mylittlewordland.com
Installation path: [hidden]
Queue driver: sync
Session driver: file
Scheduler status: Never run
Mail driver: [hidden]
Debug mode: [hidden]
```

### Possible Solution

_No response_

### Additional Context

_No response_