feat(application): enforce validation and API error mapping rules

src/core/DigitalData.TaskFlow.Application/Common/Exceptions/ForbiddenApiException.cs

@@ -0,0 +1,18 @@
+using DigitalData.TaskFlow.Domain.Enums;
+
+namespace DigitalData.TaskFlow.Application.Common.Exceptions;
+
+/// <summary>
+/// Represents a 403 API exception.
+/// </summary>
+public sealed class ForbiddenApiException : ApiException
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ForbiddenApiException"/> class.
+    /// </summary>
+    /// <param name="message">Response message.</param>
+    public ForbiddenApiException(string message)
+        : base(403, MetaCode.NoAccess, message)
+    {
+    }
+}

src/core/DigitalData.TaskFlow.Application/Common/Exceptions/InvalidRequestApiException.cs

@@ -0,0 +1,18 @@
+using DigitalData.TaskFlow.Domain.Enums;
+
+namespace DigitalData.TaskFlow.Application.Common.Exceptions;
+
+/// <summary>
+/// Represents a 400 API exception.
+/// </summary>
+public sealed class InvalidRequestApiException : ApiException
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="InvalidRequestApiException"/> class.
+    /// </summary>
+    /// <param name="message">Response message.</param>
+    public InvalidRequestApiException(string message)
+        : base(400, MetaCode.InvalidRequest, message)
+    {
+    }
+}

src/core/DigitalData.TaskFlow.Application/Common/Interfaces/IDocumentRepository.cs

@@ -12,6 +12,6 @@ public interface IDocumentRepository : IRepository
     /// </summary>
     /// <param name="id">Document identifier.</param>
     /// <param name="cancellationToken">Cancellation token.</param>
-    /// <returns>Binary document payload.</returns>
-    Task<DocumentBinaryResult> GetAsync(string id, CancellationToken cancellationToken);
+    /// <returns>Binary document payload when found; otherwise <see langword="null"/>.</returns>
+    Task<DocumentBinaryResult?> GetAsync(string id, CancellationToken cancellationToken);
 }