Add Email Sending Service with Attachment Support (#3455)

# Description of Changes

Please provide a summary of the changes, including:

- **What was changed**  
- Introduced a new `EmailService` for asynchronous email delivery with
attachment support.
- Added `MailConfig` to configure a `JavaMailSender` bean using SMTP
settings from `ApplicationProperties`.
- Created `EmailController` endpoint (`/api/v1/general/send-email`) to
accept multipart/form-data requests for sending emails.
- Defined an `Email` API model to encapsulate recipient, subject, body,
and file input.
- Extended `ApplicationProperties` to include a nested `Mail` class for
SMTP host, port, username/password, and sender address.
- Updated `settings.yml.template` to include SMTP configuration
placeholders.
- Enhanced `.github/labeler-config.yml` to cover all new security- and
API-related source files for automated labeling.

- **Why the change was made**  
- To enable Stirling-PDF to notify users via email—particularly useful
for sending generated PDFs or alerts—directly from the application.
- To centralize mail server configuration in application properties and
streamline onboarding for new environments.

---

## 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/test/java/stirling/software/SPDF/config/security/mail/EmailServiceTest.java

@@ -0,0 +1,64 @@
+package stirling.software.SPDF.config.security.mail;
+
+import static org.mockito.Mockito.*;
+
+import jakarta.mail.MessagingException;
+import jakarta.mail.internet.MimeMessage;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.mail.javamail.JavaMailSender;
+import org.springframework.mail.javamail.MimeMessageHelper;
+import org.springframework.web.multipart.MultipartFile;
+import stirling.software.SPDF.model.ApplicationProperties;
+import stirling.software.SPDF.model.api.Email;
+
+@ExtendWith(MockitoExtension.class)
+public class EmailServiceTest {
+
+    @Mock
+    private JavaMailSender mailSender;
+
+    @Mock
+    private ApplicationProperties applicationProperties;
+
+    @Mock
+    private ApplicationProperties.Mail mailProperties;
+
+    @Mock
+    private MultipartFile fileInput;
+
+    @InjectMocks
+    private EmailService emailService;
+
+    @Test
+    void testSendEmailWithAttachment() throws MessagingException {
+        // Mock the values returned by ApplicationProperties
+        when(applicationProperties.getMail()).thenReturn(mailProperties);
+        when(mailProperties.getFrom()).thenReturn("no-reply@stirling-software.com");
+
+        // Create a mock Email object
+        Email email = new Email();
+        email.setTo("test@example.com");
+        email.setSubject("Test Email");
+        email.setBody("This is a test email.");
+        email.setFileInput(fileInput);
+
+        // Mock MultipartFile behavior
+        when(fileInput.getOriginalFilename()).thenReturn("testFile.txt");
+
+        // Mock MimeMessage
+        MimeMessage mimeMessage = mock(MimeMessage.class);
+
+        // Configure mailSender to return the mocked MimeMessage
+        when(mailSender.createMimeMessage()).thenReturn(mimeMessage);
+
+        // Call the service method
+        emailService.sendEmailWithAttachment(email);
+
+        // Verify that the email was sent using mailSender
+        verify(mailSender).send(mimeMessage);
+    }
+}

src/test/java/stirling/software/SPDF/controller/api/EmailControllerTest.java

@@ -0,0 +1,84 @@
+package stirling.software.SPDF.controller.api;
+
+import static org.mockito.Mockito.*;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
+
+import jakarta.mail.MessagingException;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.test.web.servlet.MockMvc;
+import org.springframework.test.web.servlet.setup.MockMvcBuilders;
+import org.springframework.web.multipart.MultipartFile;
+import stirling.software.SPDF.config.security.mail.EmailService;
+import stirling.software.SPDF.model.api.Email;
+
+@ExtendWith(MockitoExtension.class)
+public class EmailControllerTest {
+
+    private MockMvc mockMvc;
+
+    @Mock private EmailService emailService;
+
+    @InjectMocks private EmailController emailController;
+
+    @Mock private MultipartFile fileInput;
+
+    @BeforeEach
+    void setUp() {
+        // Set up the MockMvc instance for testing
+        mockMvc = MockMvcBuilders.standaloneSetup(emailController).build();
+    }
+
+    @Test
+    void testSendEmailWithAttachmentSuccess() throws Exception {
+        // Create a mock Email object
+        Email email = new Email();
+        email.setTo("test@example.com");
+        email.setSubject("Test Email");
+        email.setBody("This is a test email.");
+        email.setFileInput(fileInput);
+
+        // Mock the service to not throw any exception
+        doNothing().when(emailService).sendEmailWithAttachment(any(Email.class));
+
+        // Perform the request and verify the response
+        mockMvc.perform(
+                        multipart("/api/v1/general/send-email")
+                                .file("fileInput", "dummy-content".getBytes())
+                                .param("to", email.getTo())
+                                .param("subject", email.getSubject())
+                                .param("body", email.getBody()))
+                .andExpect(status().isOk())
+                .andExpect(content().string("Email sent successfully"));
+    }
+
+    @Test
+    void testSendEmailWithAttachmentFailure() throws Exception {
+        // Create a mock Email object
+        Email email = new Email();
+        email.setTo("test@example.com");
+        email.setSubject("Test Email");
+        email.setBody("This is a test email.");
+        email.setFileInput(fileInput);
+
+        // Mock the service to throw a MessagingException
+        doThrow(new MessagingException("Failed to send email"))
+                .when(emailService)
+                .sendEmailWithAttachment(any(Email.class));
+
+        // Perform the request and verify the response
+        mockMvc.perform(
+                        multipart("/api/v1/general/send-email")
+                                .file("fileInput", "dummy-content".getBytes())
+                                .param("to", email.getTo())
+                                .param("subject", email.getSubject())
+                                .param("body", email.getBody()))
+                .andExpect(status().isInternalServerError())
+                .andExpect(content().string("Failed to send email: Failed to send email"));
+    }
+}