feat: implement multi-tenancy support across knowledge services and normalize TenantId to string type.

src/NexusReader.Application/Queries/Graph/GetKnowledgeGraphQueryHandler.cs

@@ -18,20 +18,13 @@ internal sealed class GetKnowledgeGraphQueryHandler : IQueryHandler<GetKnowledge
         if (string.IsNullOrWhiteSpace(request.Text))
             return Result.Ok(new GraphDataDto());
 
-        var result = await _knowledgeService.GetGraphDataAsync(request.Text, cancellationToken);
+        var result = await _knowledgeService.GetGraphDataAsync(request.Text, request.TenantId, cancellationToken);
 
         if (result.IsFailed)
             return Result.Fail<GraphDataDto>(result.Errors);
 
         var graph = result.Value.Graph;
 
-        if (graph is null)
-            return Result.Ok(new GraphDataDto());
-
-        if (graph is null)
-            return Result.Ok(new GraphDataDto());
-            
-        return Result.Ok(graph);
+        return graph is null ? Result.Ok(new GraphDataDto()) : Result.Ok(graph);
     }
 }
-