### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

Once a budget window (`budget_limits`) is set on a key, there is no way to remove it — neither through the UI nor the API. All removal attempts either silently fail or return a validation error.

**UI behavior:** - Set a budget window via the UI → works and enforces - Click X to remove the budget window → Save → budget window still present in UI and still enforces

**API behavior:**

| Attempt | Request Body | HTTP Status | Result | |---------|-------------|-------------|--------| | Empty list | `{"key": "sk-...", "budget_limits": []}` | 200 | Budget window still present (silently ignored) | | Null | `{"key": "sk-...", "budget_limits": null}` | 400 | Validation error: "Unable to match input value to any allowed input type" | | Omit field | `{"key": "sk-...", "max_budget": 999}` | 200 | Budget window still present |

The budget window continues to enforce after all removal attempts.

**Workaround:** Set the `max_budget` in the budget window to a very large number, or create a new key.

### Steps to Reproduce

1. Create a team and generate a key:…