#[entities] should error when put on a enum variant

hello !

## Bevy version

bevy 0.16.0

If your bug is rendering-related, copy the adapter info that appears when you run Bevy.

## What you did
```rust
#[derive(Component)]
pub enum TroopState {
Idle,
#[entities] // should error
Run(Option),
#[entities] // should error
Attack(Target),
}
#[derive(Component)]
pub struct Target {
#[entities]
pub entity: Entity,
}
```
`Target` is not mapped, this is a footgun, without any warning

## What went wrong

the `#[entities]` should be on the associated type like so `Run(#[entities] Option)`
maybe the derive macro can error when the annotation is on the enum variant instead of actual type ?

Start by reproducing the enum example with #[derive(Component)] and the #[entities] annotations on variants and fields. Inspect the Component derive handling for enum variants; done means an annotation placed on a variant produces an error while the annotation on the associated type remains accepted.

Written by the indexing model from the issue text.