fix(ui): ensure buildFormStateHandler throws error instead of returning null for unauthorized requests (#13123)

### What?

Prevents `buildFormStateHandler` from returning `null` in unauthorized
scenarios by throwing an explicit `Error` instead.

### Why?

The `BuildFormStateResult` type does not include `null`, but previously
the handler returned `null` when access was unauthorized. This caused
runtime type mismatches and forced client-side workarounds (e.g.
guarding destructures).

By always throwing instead of returning `null`, the client code can
safely assume a valid result or catch errors.

<img width="1772" height="723" alt="Screenshot_2025-07-10_185618"
src="https://github.com/user-attachments/assets/d65344e3-a2cb-4ec5-91bf-a353b5b7dd14"
/>

### How?

- Replaced the `return null` with `throw new Error('Unauthorized')` in
`buildFormStateHandler`.
- Client code no longer needs to handle `null` responses from
`getFormState`.

packages/ui/src/utilities/buildFormState.ts

@@ -94,7 +94,7 @@ export const buildFormStateHandler: ServerFunction<
     }
 
     if (err.message === 'Unauthorized') {
-      return null
+      throw new Error('Unauthorized')
     }
 
     return formatErrors(err)