Fix: Session of admin is destroyed instead of the deleted user (#3218)

# Description of Changes

Please provide a summary of the changes, including:

- Replaced `authentication.getPrincipal()` with `username` in the
`sessionRegistry.getAllSessions(...)` call inside the `deleteUser`
method of `UserController`.
- The original implementation incorrectly used the currently
authenticated principal to fetch sessions, which could lead to only
invalidating the sessions of the user performing the deletion — not the
target user being deleted.
- By using the `username` parameter directly, this ensures **all
sessions of the user being deleted are properly expired and removed**.

Closes #(issue_number)

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.

src/main/java/stirling/software/SPDF/controller/api/UserController.java

@@ -333,7 +333,7 @@ public class UserController {
         }
         // Invalidate all sessions before deleting the user
         List<SessionInformation> sessionsInformations =
-                sessionRegistry.getAllSessions(authentication.getPrincipal(), false);
+                sessionRegistry.getAllSessions(username, false);
         for (SessionInformation sessionsInformation : sessionsInformations) {
             sessionRegistry.expireSession(sessionsInformation.getSessionId());
             sessionRegistry.removeSessionInformation(sessionsInformation.getSessionId());