Fix ManagedBy type to be more type-safe

Currently, if you hover over `ManagedBy` in VSCode, you'll get `string`. This is not what we want. We want `ManagedBy` to be equal to `"vercel-marketplace" | "aws-marketplace" | "supabase"`. A simple `as const` in front of `MANAGED_BY` fixed the issue. For example: <img width="452" height="217" alt="Image" src="https://github.com/user-attachments/assets/527177a8-946f-4d29-a2fb-926e52cd87eb" /> If you hover over `ToolCategory`, you'll see that it equals `"data" | "schema" | "log" | "ui"`, not just `string`, thanks to `as const` after `TOOL_CATEGORIES`. That's what I did with `ManagedBy`.