Refactor EmailMessage and tests for simplified initialization

Implemented property initializers in EmailSender and EmailRecipient using primary constructors for a more concise and clear initialization. Refactored relevant test methods to use expression-bodied members for consistency and improved readability.

src/MailFusion/EmailMessage.cs

@@ -1,3 +1,5 @@
+using System.Diagnostics.CodeAnalysis;
+
 namespace MailFusion;
 
 /// <summary>
@@ -209,12 +211,9 @@ public EmailSender()
     /// );
     /// </code>
     /// </example>
+    [SetsRequiredMembers]
     public EmailSender(string email, string name, string? replyEmail = null)
-    {
-        Email = email;
-        Name = name;
-        ReplyEmail = replyEmail ?? email;
-    }
+        => (Email, Name, ReplyEmail) = (email, name, replyEmail ?? email);
 }
 
 /// <summary>
@@ -310,9 +309,6 @@ public EmailRecipient()
     /// );
     /// </code>
     /// </example>
-    public EmailRecipient(string email, string? name = null)
-    {
-        Email = email;
-        Name = name;
-    }
+    [SetsRequiredMembers]
+    public EmailRecipient(string email, string? name = null) => (Email, Name) = (email, name);
 }

test/MailFusion.Tests/EmailServiceTests.cs

@@ -98,42 +98,36 @@ public async Task SendFromTemplateAsync_WithEmptyRecipients_ShouldReturnFailure(
         result.Error!.Code.Should().Be(EmailServiceErrors.Codes.InvalidInput);
     }
 
-    private static EmailMessage CreateValidEmailMessage()
-    {
-        return new EmailMessage
+    private static EmailMessage CreateValidEmailMessage() =>
+        new()
         {
             Subject = "Test Subject",
             HtmlBody = "<p>Test Body</p>",
             PlainTextBody = "Test Body",
             Sender = CreateValidSender(),
             Recipients = CreateValidRecipients()
         };
-    }
 
-    private static EmailSender CreateValidSender()
-    {
-        return new EmailSender
+    private static EmailSender CreateValidSender() =>
+        new()
         {
             Name = "Test Sender",
             Email = "sender@test.com",
             ReplyEmail = "reply@test.com"
         };
-    }
 
-    private static List<EmailRecipient> CreateValidRecipients()
-    {
-        return [new EmailRecipient { Email = "recipient@test.com", Name = "Test Recipient" }];
-    }
+    private static List<EmailRecipient> CreateValidRecipients() =>
+    [
+        new("recipient@test.com", "Test Recipient")
+    ];
 
-    private static EmailTemplate CreateValidTemplate()
-    {
-        return new EmailTemplate
+    private static EmailTemplate CreateValidTemplate() =>
+        new()
         {
             Subject = "Test Subject",
             HtmlBody = "<p>Test Body</p>",
             PlainTextBody = "Test Body"
         };
-    }
 
     private class TestEmailModel : IEmailTemplateModel
     {