fix(saml): correct ClassPathResource handling for IdP metadata and add null-guard for privateKey (#4157)

## Description of Changes

**What was changed**
- In `getIdpMetadataUri()`, use
`idpMetadataUri.substring("classpath:".length())` so the `classpath:`
scheme (including the colon) is stripped correctly before creating the
`ClassPathResource`.
- In `getPrivateKey()`, add a null check (`if (privateKey == null)
return null;`) to avoid a potential `NullPointerException` when the
property is unset.

**Why the change was made**
- The previous substring used `"classpath".length()` (without the
colon), leaving a leading `:` in the path (e.g., `:/saml/idp.xml`) which
breaks `ClassPathResource` resolution and can prevent SAML bootstrapping
when `idpMetadataUri` uses the `classpath:` scheme.
- The null-guard aligns the method with defensive coding practices and
prevents runtime errors when no private key is configured.


---

## 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/devGuide/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/devGuide/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/devGuide/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/devGuide/DeveloperGuide.md#6-testing)
for more details.

app/common/src/main/java/stirling/software/common/model/ApplicationProperties.java

@@ -197,7 +197,7 @@ public class ApplicationProperties {
             @JsonIgnore
             public InputStream getIdpMetadataUri() throws IOException {
                 if (idpMetadataUri.startsWith("classpath:")) {
-                    return new ClassPathResource(idpMetadataUri.substring("classpath".length()))
+                    return new ClassPathResource(idpMetadataUri.substring("classpath:".length()))
                             .getInputStream();
                 }
                 try {
@@ -233,6 +233,7 @@ public class ApplicationProperties {
 
             @JsonIgnore
             public Resource getPrivateKey() {
+                if (privateKey == null) return null;
                 if (privateKey.startsWith("classpath:")) {
                     return new ClassPathResource(privateKey.substring("classpath:".length()));
                 } else {