add completions methods (not final)

SurveyBackend/SurveyBackend.Services/Services/CompletionService.cs

@@ -1,3 +1,4 @@
+using SurveyBackend.Services.Exceptions;
 using SurveyLib.Core.Models;
 using SurveyLib.Core.Repositories;
 using SurveyLib.Core.Services;
@@ -31,6 +32,17 @@ public class CompletionService : ICompletionService
         await _completionRepository.DeleteAsync(id);
     }
 
+    public async Task<Completion> GetCompletionByIdAsync(int id)
+    {
+        var completion = await _completionRepository.GetByIdAsync(id);
+        if (completion is null)
+        {
+            throw new NotFoundException("Completion not found");
+        }
+
+        return completion;
+    }
+
     public async Task<IEnumerable<Completion>> GetCompletionsBySurveyIdAsync(int surveyId)
     {
         // TODO: проверить существование опроса