Improve Type-Safe Casting with Pattern Matching (#2990)

# Description of Changes

Please provide a summary of the changes, including:

This PR refactors multiple instances of type casting throughout the
codebase by replacing them with Java's pattern matching for
`instanceof`. This approach eliminates redundant type casting, improves
code readability, and reduces the chances of `ClassCastException`. The
changes primarily affect authentication handling, PDF processing, and
certificate validation.

### Key Changes:
- Replaced traditional `instanceof` checks followed by explicit casting
with pattern matching.
- Improved readability and maintainability of type-related operations.
- Applied changes across security modules, PDF utilities, and image
processing functions.

This refactor does not introduce new functionality but enhances the
robustness and clarity of the existing code.

pending until #2818 is published

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] 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)
- [ ] I have performed a self-review of my own code
- [ ] 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/config/security/UserService.java

@@ -385,14 +385,14 @@ public class UserService implements UserServiceInterface {
         for (Object principal : sessionRegistry.getAllPrincipals()) {
             for (SessionInformation sessionsInformation :
                     sessionRegistry.getAllSessions(principal, false)) {
-                if (principal instanceof UserDetails userDetails) {
-                    usernameP = userDetails.getUsername();
+                if (principal instanceof UserDetails detailsUser) {
+                    usernameP = detailsUser.getUsername();
                 } else if (principal instanceof OAuth2User oAuth2User) {
                     usernameP = oAuth2User.getName();
                 } else if (principal instanceof CustomSaml2AuthenticatedPrincipal saml2User) {
                     usernameP = saml2User.name();
-                } else if (principal instanceof String) {
-                    usernameP = (String) principal;
+                } else if (principal instanceof String stringUser) {
+                    usernameP = stringUser;
                 }
                 if (usernameP.equalsIgnoreCase(username)) {
                     sessionRegistry.expireSession(sessionsInformation.getSessionId());
@@ -404,17 +404,17 @@ public class UserService implements UserServiceInterface {
     public String getCurrentUsername() {
         Object principal = SecurityContextHolder.getContext().getAuthentication().getPrincipal();
 
-        if (principal instanceof UserDetails) {
-            return ((UserDetails) principal).getUsername();
-        } else if (principal instanceof OAuth2User) {
-            return ((OAuth2User) principal)
-                    .getAttribute(
-                            applicationProperties.getSecurity().getOauth2().getUseAsUsername());
-        } else if (principal instanceof CustomSaml2AuthenticatedPrincipal) {
-            return ((CustomSaml2AuthenticatedPrincipal) principal).name();
-        } else {
-            return principal.toString();
+        if (principal instanceof UserDetails detailsUser) {
+            return detailsUser.getUsername();
+        } else if (principal instanceof OAuth2User oAuth2User) {
+            return oAuth2User.getAttribute(
+                    applicationProperties.getSecurity().getOauth2().getUseAsUsername());
+        } else if (principal instanceof CustomSaml2AuthenticatedPrincipal saml2User) {
+            return saml2User.name();
+        } else if (principal instanceof String stringUser) {
+            return stringUser;
         }
+        return null;
     }
 
     @Transactional