Date and time placeholders in addTextStamp are not working (#5446)

# Description of Changes

The processedStampText variable should be used as parameter when
splitting text into lines. This never happened so date and time
placeholder were never used.

---

## 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)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### 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/core/src/main/java/stirling/software/SPDF/controller/api/misc/StampController.java

@@ -273,7 +273,7 @@ public class StampController {
 
         // Split the stampText into multiple lines
         String[] lines =
-                RegexPatternUtils.getInstance().getEscapedNewlinePattern().split(stampText);
+                RegexPatternUtils.getInstance().getEscapedNewlinePattern().split(processedStampText);
 
         // Calculate dynamic line height based on font ascent and descent
         float ascent = font.getFontDescriptor().getAscent();