Enhance email error handling and expand test coverage (#3561)

# Description of Changes

Please provide a summary of the changes, including:

- **What was changed**  
- **EmailController**: Added a `catch (MailSendException)` block to
handle invalid-address errors, log the exception, and return a 500
response with the raw error message.
- **EmailServiceTest**: Added unit tests for attachment-related error
cases (missing filename, null filename, missing file, null file) and
invalid “to” address (null or empty), expecting `MessagingException` or
`MailSendException`.
- **MailConfigTest**: New test class verifying `MailConfig.java`
correctly initializes `JavaMailSenderImpl` with host, port, username,
password, default encoding, and SMTP properties.
- **EmailControllerTest**: Refactored into a parameterized test
(`shouldHandleEmailRequests`) covering four scenarios: success, generic
messaging error, missing `to` parameter, and invalid address formatting.

- **Why the change was made**  
- To ensure invalid email addresses and missing attachments are handled
gracefully at the controller layer, providing clearer feedback to API
clients.
- To improve overall test coverage and guard against regressions in
email functionality.
  - To enforce correct mail configuration via automated tests.

---

## 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
- [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/mail/EmailService.java

@@ -37,8 +37,21 @@ public class EmailService {
      */
     @Async
     public void sendEmailWithAttachment(Email email) throws MessagingException {
-        ApplicationProperties.Mail mailProperties = applicationProperties.getMail();
         MultipartFile file = email.getFileInput();
+        // 1) Validate recipient email address
+        if (email.getTo() == null || email.getTo().trim().isEmpty()) {
+            throw new MessagingException("Invalid Addresses");
+        }
+
+        // 2) Validate attachment
+        if (file == null
+                || file.isEmpty()
+                || file.getOriginalFilename() == null
+                || file.getOriginalFilename().isEmpty()) {
+            throw new MessagingException("An attachment is required to send the email.");
+        }
+
+        ApplicationProperties.Mail mailProperties = applicationProperties.getMail();
 
         // Creates a MimeMessage to represent the email
         MimeMessage message = mailSender.createMimeMessage();

src/main/java/stirling/software/SPDF/controller/api/EmailController.java

@@ -3,6 +3,7 @@ package stirling.software.SPDF.controller.api;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
+import org.springframework.mail.MailSendException;
 import org.springframework.web.bind.annotation.ModelAttribute;
 import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
@@ -53,6 +54,11 @@ public class EmailController {
             // Calls the service to send the email with attachment
             emailService.sendEmailWithAttachment(email);
             return ResponseEntity.ok("Email sent successfully");
+        } catch (MailSendException ex) {
+            // handles your "Invalid Addresses" case
+            String errorMsg = ex.getMessage();
+            log.error("MailSendException: {}", errorMsg, ex);
+            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(errorMsg);
         } catch (MessagingException e) {
             // Catches any messaging exception (e.g., invalid email address, SMTP server issues)
             String errorMsg = "Failed to send email: " + e.getMessage();