`World::spawn_scene` should not require `AssetServer`.

[mintlu8]

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

World::spawn_scene is good for testing purpose but pulling in AssetServer requires a lot of setup.

What solution would you like?

Make World::spawn_scene not panic if AssetServer is missing.

[webdevsamran]

Hi team, I would like to work on making World::spawn_scene more flexible for testing.

Currently World::spawn_scene panics if AssetServer is not present in the world, which complicates isolated unit tests. I can update spawn_scene to handle missing AssetServer resources gracefully when scene assets are already loaded into Assets<DynamicScene>, and add unit test coverage in bevy_scene.

Could you please assign this to me? Thanks!