feat(api): manage localized push template definitions

Add revision-protected reads, atomic upsert and deletion with structured actions. Preserve omitted translations and fix insertion of new push languages.

Ref: IT-1627

HrynCo.NotificationService.Services/TemplateDefinitions/GetPushTemplateDefinitionHandler.cs

@@ -0,0 +1,19 @@
+using HrynCo.NotificationService.DAL.Abstract.Repositories;
+using HrynCo.NotificationService.Services.Core;
+using HrynCo.NotificationService.Services.Logging;
+using static HrynCo.NotificationService.Services.Core.ServiceResultHelper;
+
+namespace HrynCo.NotificationService.Services.TemplateDefinitions;
+
+internal sealed class GetPushTemplateDefinitionHandler(
+    IContextualSerilogLogger<GetPushTemplateDefinitionQuery> logger, IPushTemplateDefinitionRepository definitions)
+    : RequestHandler<GetPushTemplateDefinitionQuery, ServiceResult<PushTemplateDefinitionDto>>(logger)
+{
+    protected override async Task<ServiceResult<PushTemplateDefinitionDto>> DoOnHandle(GetPushTemplateDefinitionQuery request, CancellationToken cancellationToken)
+    {
+        var definition = await definitions.GetAsync(request.ServiceId, request.Key, cancellationToken);
+        return definition is null
+            ? Failure<PushTemplateDefinitionDto>("Template definition not found.", ServiceErrorCode.NotFound)
+            : Success(PushTemplateDefinitionMapping.Map(definition));
+    }
+}