Dashboards: Save fails due to invisible character in data link URL

### What happened? Dashboard failed to save due to an invisible BOM / zero-width character in a data link URL. <img width="925" height="521" alt="Image" src="https://github.com/user-attachments/assets/790862a2-d0fb-438e-adef-daf5c7d2b4bf" /> ### What did you expect to happen? The dashboard should save successfully, or we should display a clear error indicating the offending field. ### Did this work before? Yes. I have been saving dashboards with URLs like this for a few years now. ### How do we reproduce it? 1. Create or edit a dashboard with a table panel 2. Add a data link whose URL contains a zero-width / BOM character at the end (e.g. U+FEFF) 3. Save the dashboard. 4. It fails! ### Is the bug inside a dashboard panel? _No response_ ### Environment (with versions)? Grafana: Cloud Fast OS: macOS Browser: Chrome ### Grafana platform? I use Grafana Cloud ### Datasource(s)? _No response_