Validate H2 Database Type and URL Consistency for Custom Databases (#3458)

# Description of Changes

Please provide a summary of the changes, including:

- **What was changed**  
Introduced a local `isCustomDatabase` flag (based on
`datasource.isEnableCustomDatabase()`) to ensure that the H2-specific
URL/type consistency checks (and corresponding warnings/exceptions) only
run when a custom database configuration is enabled. Refactored the
return statement to use this flag (`return !isCustomDatabase || isH2;`)
instead of calling `isEnableCustomDatabase()` directly.

- **Why the change was made**  
Previously, even when custom database support was disabled, the method
would still validate H2 configuration and potentially throw an
`IllegalStateException`. By guarding those checks, we avoid spurious
warnings or exceptions in default (non-custom) setups and make the
method’s behavior more predictable.

---

## 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)
- [ ] 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/config/security/database/DatabaseService.java

@@ -246,14 +246,16 @@ public class DatabaseService implements DatabaseInterface {
         boolean isDBUrlH2 =
                 datasource.getCustomDatabaseUrl().contains("h2")
                         || datasource.getCustomDatabaseUrl().contains("H2");
+        boolean isCustomDatabase = datasource.isEnableCustomDatabase();
 
+        if (isCustomDatabase) {
             if (isTypeH2 && !isDBUrlH2) {
                 log.warn(
                         "Datasource type is H2, but the URL does not contain 'h2'. "
                                 + "Please check your configuration.");
                 throw new IllegalStateException(
-                    "Datasource type is H2, but the URL does not contain 'h2'. Please check your"
+                        "Datasource type is H2, but the URL does not contain 'h2'. Please check"
-                            + " configuration.");
+                                + " your configuration.");
             } else if (!isTypeH2 && isDBUrlH2) {
                 log.warn(
                         "Datasource URL contains 'h2', but the type is not H2. "
@@ -262,10 +264,10 @@ public class DatabaseService implements DatabaseInterface {
                         "Datasource URL contains 'h2', but the type is not H2. Please check your"
                                 + " configuration.");
             }
-
+        }
         boolean isH2 = isTypeH2 && isDBUrlH2;
 
-        return !datasource.isEnableCustomDatabase() || isH2;
+        return !isCustomDatabase || isH2;
     }
 
     /**