Eml-to-pdf bug fixes: removal of incompatible fonts, removal of emoji in favor of @, jakarta-mail dependency handling improvements (#3770)

# Description of Changes
This pull request introduces enhancements and code cleanup to the
`EmlToPdf` utility class, focusing on improving email-to-PDF conversion,
handling embedded images, and simplifying the codebase. Key changes
include better handling of inline images, enhanced Jakarta Mail
dependency checks, and refactoring for improved readability and
maintainability.

### Enhancements to Email-to-PDF Conversion:
* Added support for processing inline images (`cid:` references) by
converting them into data URIs for proper inline display.
* Improved attachment handling to always include embedded images
regardless of size, ensuring inline display functionality.
* Enhanced email HTML generation to process inline images and include
them in the email body.

### Attachment Handling Enhancements:
* Replaced the attachment icon placeholder (`icon` or 📎 emoji) with a
new marker (`@`) for consistency across the application (non-fat images
did not support the emoji, however @ is supported accross the board.)
* Updated the annotation logic to use `AttachmentMarkerPositionFinder`
instead of `EmojiPositionFinder`, aligning with the new attachment
marker system.


### Jakarta Mail Dependency Handling:
* Added detailed checks for core Jakarta Mail classes to determine
availability in different environments (e.g., Docker).
* Introduced validation for Jakarta Mail multipart and part types to
prevent processing invalid objects.
* Explicitly parse in the classes:

- jakarta.mail.internet.MimeMessage – Core email message parsing
- jakarta.mail.Session – Email session management
- jakarta.mail.internet.MimeUtility – MIME encoding/decoding utilities
- jakarta.mail.internet.MimePart – Individual MIME parts (attachments,
body parts)
- jakarta.mail.internet.MimeMultipart – Multi-part MIME messages
- jakarta.mail.Multipart – Base multipart interface
- jakarta.mail.Part – Base part interface

### Code Cleanup and Refactoring:
* Simplified utility classes (`StyleConstants`, `MimeConstants`,
`FileSizeConstants`) by removing unnecessary constructors and unused
constants.
* Updated log messages for clarity, such as distinguishing between
general content processing errors and multipart-specific issues.
---

## 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)
- [x] 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)

- [x] 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.

stirling-pdf/src/main/java/stirling/software/SPDF/config/EndpointConfiguration.java

@@ -10,7 +10,6 @@ import org.springframework.beans.factory.annotation.Qualifier;
 import org.springframework.stereotype.Service;
 
 import lombok.extern.slf4j.Slf4j;
-
 import stirling.software.common.model.ApplicationProperties;
 
 @Service
@@ -142,6 +141,7 @@ public class EndpointConfiguration {
         addEndpointToGroup("Convert", "markdown-to-pdf");
         addEndpointToGroup("Convert", "pdf-to-csv");
         addEndpointToGroup("Convert", "pdf-to-markdown");
+        addEndpointToGroup("Convert", "eml-to-pdf");
 
         // Adding endpoints to "Security" group
         addEndpointToGroup("Security", "add-password");
@@ -265,6 +265,7 @@ public class EndpointConfiguration {
         addEndpointToGroup("Weasyprint", "html-to-pdf");
         addEndpointToGroup("Weasyprint", "url-to-pdf");
         addEndpointToGroup("Weasyprint", "markdown-to-pdf");
+        addEndpointToGroup("Weasyprint", "eml-to-pdf");
 
         // Pdftohtml dependent endpoints
         addEndpointToGroup("Pdftohtml", "pdf-to-html");