Refactor: Replace EmailAccountDto with EmailAccount

Replaced the `EmailAccountDto` class with the `EmailAccount` class across the codebase to consolidate the `EmailAccount` entity into the domain layer. Updated namespaces, method signatures, property types, and test cases to reflect this change.

Moved `EmailAccount` from `DigitalData.MessagingService.Application.Common.Dto` to `DigitalData.MessagingService.Domain.Entities`. Updated XML documentation and removed redundant project file entries. Adjusted namespaces for related queries, validators, and commands to align with the new structure.

These changes improve separation of concerns and align with domain-driven design principles.

src/core/DigitalData.MessagingService.Domain/Entities/EmailAccount.cs

@@ -0,0 +1,52 @@
+namespace DigitalData.MessagingService.Domain.Entities;
+
+/// <summary>
+/// DTO for a single email account configuration.
+/// </summary>
+public class EmailAccount
+{
+    /// <summary>
+    /// Logical name to identify this account (e.g. "default", "support").
+    /// </summary>
+    public int Id { get; set; }
+
+#if NET
+    public required string Username { get; set; }
+#else
+    public string Username { get; set; } = null!;
+#endif
+
+#if NET
+    public required string Password { get; set; }
+#else
+    public string Password { get; set; } = null!;
+#endif
+
+#if NET
+    public required string SmtpServer { get; set; }
+#else
+    public string SmtpServer { get; set; } = null!;
+#endif
+
+    public int SmtpPort { get; set; }
+
+    public bool SmtpUseSsl { get; set; }
+
+    public bool UseOAuth2 { get; set; }
+
+    /// <summary>
+    /// IMAP server hostname (e.g. "imap.example.com").
+    /// Leave empty when this account is send-only.
+    /// </summary>
+    public string? ImapServer { get; set; }
+
+    /// <summary>
+    /// IMAP server port (993 for SSL, 143 for plain/STARTTLS).
+    /// </summary>
+    public int ImapPort { get; set; } = 993;
+
+    /// <summary>
+    /// Use SSL/TLS when connecting to the IMAP server.
+    /// </summary>
+    public bool ImapUseSsl { get; set; } = true;
+}