Spawning/inserting a resource should replace existing resources instead of bouncing

## What problem does this solve or what need does it fill?

When spawning/inserting an already existing resource it will error.

```
2026-05-07T20:04:32.358321Z WARN bevy_ecs::resource: Tried inserting the resource while one already exists.
Resources are unique components stored on a single entity.
Inserting on a different entity, when one already exists, causes the new value to be removed.
```

However this doesn't align with the current behaviour of `init_resource`/`insert_resource`.

This makes spawning resources with extra components difficult.

```rust
// This works perfectly.
commands.insert_resource(Level(1));

// But this doesn't
commands.spawn((
Name::new("Level"),
Level(2)
));

// unless you remove the resource first.
commands.remove_resource::();
commands.spawn((
Name::new("Level"),
Level(3)
));
```

## What solution would you like?

Either

- Change `init_resource`/`insert_resource` to fail if it already exists
- Change `spawn(Resource)` to replace the existing resource.

Start by tracing the Commands APIs named in the issue: insert_resource, init_resource, spawn, and remove_resource, then compare how each handles an existing resource. Done means the chosen behavior is consistent for direct insertion and spawning with extra components, with coverage for the existing-resource case.

Written by the indexing model from the issue text.