Fix sparse set components ignoring insert_if_new/InsertMode

[JaySpruce]

Objective

I've been tinkering with ECS insertion/removal lately, and noticed that sparse sets just... don't interact with InsertMode at all. Sure enough, using insert_if_new with a sparse component does the same thing as insert.

Solution

• Add a check in BundleInfo::write_components to drop the new value if the entity already has the component and InsertMode is Keep.
• Add necessary methods to sparse set internals to fetch the drop function.

Testing

Minimal reproduction:

Code

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(PostStartup, component_print)
        .run();
}

#[derive(Component)]
#[component(storage = "SparseSet")]
struct SparseComponent(u32);

fn setup(mut commands: Commands) {
    let mut entity = commands.spawn_empty();
    entity.insert(SparseComponent(1));
    entity.insert(SparseComponent(2));

    let mut entity = commands.spawn_empty();
    entity.insert(SparseComponent(3));
    entity.insert_if_new(SparseComponent(4));
}

fn component_print(query: Query<&SparseComponent>) {
    for component in &query {
        info!("{}", component.0);
    }
}

Here it is on Bevy Playground (0.15.3):
https://learnbevy.com/playground?share=2a96a68a81e804d3fdd644a833c1d51f7fa8dd33fc6192fbfd077b082a6b1a41

Output on main:

2025-05-04T17:50:50.401328Z  INFO system{name="fork::component_print"}: fork: 2
2025-05-04T17:50:50.401583Z  INFO system{name="fork::component_print"}: fork: 4

Output with this PR :

2025-05-04T17:51:33.461835Z  INFO system{name="fork::component_print"}: fork: 2
2025-05-04T17:51:33.462091Z  INFO system{name="fork::component_print"}: fork: 3

[chescock]

Should we add a unit test that would have caught this?