Add CloseAsync method to WorkflowRepository

Introduced a new asynchronous method `CloseAsync` in the
`WorkflowRepository` class to handle workflow process closing.
This method executes the `PRCUST_F19_CLOSING` stored procedure
with parameters for user ID, source, process ID, closing method
ID, comment, and rejection reason ID. It uses `FromSqlInterpolated`
for safe SQL parameterization and retrieves results with
`AsNoTracking()` and `ToListAsync(cancellationToken)`. The method
returns the first result as a `WorkflowClosing` object or `null`.

src/infrastructure/DigitalData.TaskFlow.Infrastructure/Repositories/WorkflowRepository.cs

@@ -19,4 +19,35 @@ public sealed class WorkflowRepository(TaskFlowDbContext context) : IWorkflowRep
 
         return rows.FirstOrDefault();
     }
+
+    public async Task<WorkflowClosing?> CloseAsync(
+        int userId,
+        string source,
+        long processId,
+        int closingMethodId,
+        string? comment,
+        byte? rejectionReasonId,
+        CancellationToken cancellationToken)
+    {
+        var rows = await context.WorkflowClosings
+            .FromSqlInterpolated($@"
+DECLARE @ResultCode    INT;
+DECLARE @ResultMessage NVARCHAR(400);
+
+EXEC dbo.PRCUST_F19_CLOSING
+    @UserID            = {userId},
+    @Source            = {source},
+    @ProcessID         = {processId},
+    @ClosingMethodID   = {closingMethodId},
+    @Comment           = {comment},
+    @RejectionReasonID = {rejectionReasonId},
+    @ResultCode        = @ResultCode OUTPUT,
+    @ResultMessage     = @ResultMessage OUTPUT;
+
+SELECT @ResultCode AS CODE, @ResultMessage AS RESULT;")
+            .AsNoTracking()
+            .ToListAsync(cancellationToken);
+
+        return rows.FirstOrDefault();
+    }
 }