Dashboard variables: combining lists of variables contains unnecessary curly braces

### What happened? I have two dashboard variables based on a query. When adding a third custom variable with the data of the previous two variables, the first and last values from the query variables contain curly braces which shouldn't be there. The basic idea was described here: https://community.grafana.com/t/populate-dashboard-variable-from-2-queries/157352/2 ### What did you expect to happen? Having a combined list without additional curly braces in some of the values. ### Did this work before? I don't know if this every worked correctly with with the given settings. ### How do we reproduce it? 1. Create a variable based on a query 2. (optional) Create another variable based on a query 3. Create a custom variable which is based on 1. and 2. First query variable: <img width="721" height="1206" alt="Image" src="https://github.com/user-attachments/assets/b6743c3e-68d6-4871-8fbc-0490fb11e3bf" /> Second query variable: <img width="500" height="1210" alt="Image" src="https://github.com/user-attachments/assets/06ef084d-c760-4c56-bd53-263f4f0271f1" /> Combined custom variable: <img width="813" height="854" alt="Image" src="https://github.com/user-attachments/assets/a852b2f0-4b27-4928-becd-767fcf573f7e" /> ### Is the bug inside a dashboard panel? _No response_ ### Environment (with versions)? Grafana: Grafana v12.1.1 (df5de8219b) ### Grafana platform? None ### Datasource(s)? _No response_