Add CloseWorkflowCommandHandler for workflow closure

Introduce CloseWorkflowCommandHandler class to handle the
closure of workflows using the MediatR library. This class
implements IRequestHandler and processes CloseWorkflowCommand
requests, returning CloseWorkflowResult objects. It interacts
with IWorkflowRepository to execute the CloseAsync method,
validating the procedure result and throwing an exception if
the result is invalid. Necessary using directives for
DigitalData.TaskFlow.Application.Common.Interfaces and MediatR
are also added.

src/core/DigitalData.TaskFlow.Application/Features/Workflows/Commands/CloseWorkflowCommandHandler.cs

@@ -0,0 +1,32 @@
+using DigitalData.TaskFlow.Application.Common.Interfaces;
+using MediatR;
+
+namespace DigitalData.TaskFlow.Application.Features.Workflows.Commands;
+
+public sealed class CloseWorkflowCommandHandler(IWorkflowRepository repo)
+    : IRequestHandler<CloseWorkflowCommand, CloseWorkflowResult>
+{
+    public async Task<CloseWorkflowResult> Handle(
+        CloseWorkflowCommand request,
+        CancellationToken cancellationToken)
+    {
+        var procedureResult = await repo.CloseAsync(
+            userId: request.WisagId,
+            source: request.Source,
+            processId: request.ProcessId,
+            closingMethodId: request.ClosingMethodId,
+            comment: request.Comment,
+            rejectionReasonId: request.RejectionReasonId,
+            cancellationToken: cancellationToken);
+
+        if (procedureResult?.Code is null)
+        {
+            throw new InvalidOperationException(
+                "Stored procedure returned an invalid close result.");
+        }
+
+        return new CloseWorkflowResult(
+            ResultCode: procedureResult.Code.Value,
+            ResultMessage: procedureResult.Result ?? string.Empty);
+    }
+}