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.