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/session/SessionPersistentRegistry.java

@@ -43,14 +43,14 @@ public class SessionPersistentRegistry implements SessionRegistry {
         List<SessionInformation> sessionInformations = new ArrayList<>();
         String principalName = null;
 
-        if (principal instanceof UserDetails) {
-            principalName = ((UserDetails) principal).getUsername();
-        } else if (principal instanceof OAuth2User) {
-            principalName = ((OAuth2User) principal).getName();
-        } else if (principal instanceof CustomSaml2AuthenticatedPrincipal) {
-            principalName = ((CustomSaml2AuthenticatedPrincipal) principal).name();
-        } else if (principal instanceof String) {
-            principalName = (String) principal;
+        if (principal instanceof UserDetails detailsUser) {
+            principalName = detailsUser.getUsername();
+        } else if (principal instanceof OAuth2User oAuth2User) {
+            principalName = oAuth2User.getName();
+        } else if (principal instanceof CustomSaml2AuthenticatedPrincipal saml2User) {
+            principalName = saml2User.name();
+        } else if (principal instanceof String stringUser) {
+            principalName = stringUser;
         }
 
         if (principalName != null) {
@@ -74,14 +74,14 @@ public class SessionPersistentRegistry implements SessionRegistry {
     public void registerNewSession(String sessionId, Object principal) {
         String principalName = null;
 
-        if (principal instanceof UserDetails) {
-            principalName = ((UserDetails) principal).getUsername();
-        } else if (principal instanceof OAuth2User) {
-            principalName = ((OAuth2User) principal).getName();
-        } else if (principal instanceof CustomSaml2AuthenticatedPrincipal) {
-            principalName = ((CustomSaml2AuthenticatedPrincipal) principal).name();
-        } else if (principal instanceof String) {
-            principalName = (String) principal;
+        if (principal instanceof UserDetails detailsUser) {
+            principalName = detailsUser.getUsername();
+        } else if (principal instanceof OAuth2User oAuth2User) {
+            principalName = oAuth2User.getName();
+        } else if (principal instanceof CustomSaml2AuthenticatedPrincipal saml2User) {
+            principalName = saml2User.name();
+        } else if (principal instanceof String stringUser) {
+            principalName = stringUser;
         }
 
         if (principalName != null) {