Fix sparse set components ignoring insert_if_new/InsertMode (#19059)

# 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:
<details>
<summary>Code</summary>

```
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);
    }
}
```

</details>

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
```

crates/bevy_ecs/src/bundle.rs

@@ -630,13 +630,14 @@ impl BundleInfo {
         let mut bundle_component = 0;
         let after_effect = bundle.get_components(&mut |storage_type, component_ptr| {
             let component_id = *self.component_ids.get_unchecked(bundle_component);
+            // SAFETY: bundle_component is a valid index for this bundle
+            let status = unsafe { bundle_component_status.get_status(bundle_component) };
             match storage_type {
                 StorageType::Table => {
-                    // SAFETY: bundle_component is a valid index for this bundle
-                    let status = unsafe { bundle_component_status.get_status(bundle_component) };
-                    // SAFETY: If component_id is in self.component_ids, BundleInfo::new ensures that
-                    // the target table contains the component.
-                    let column = table.get_column_mut(component_id).debug_checked_unwrap();
+                    let column =
+                        // SAFETY: If component_id is in self.component_ids, BundleInfo::new ensures that
+                        // the target table contains the component.
+                        unsafe { table.get_column_mut(component_id).debug_checked_unwrap() };
                     match (status, insert_mode) {
                         (ComponentStatus::Added, _) => {
                             column.initialize(table_row, component_ptr, change_tick, caller);
@@ -656,7 +657,16 @@ impl BundleInfo {
                         // SAFETY: If component_id is in self.component_ids, BundleInfo::new ensures that
                         // a sparse set exists for the component.
                         unsafe { sparse_sets.get_mut(component_id).debug_checked_unwrap() };
-                    sparse_set.insert(entity, component_ptr, change_tick, caller);
+                    match (status, insert_mode) {
+                        (ComponentStatus::Added, _) | (_, InsertMode::Replace) => {
+                            sparse_set.insert(entity, component_ptr, change_tick, caller);
+                        }
+                        (ComponentStatus::Existing, InsertMode::Keep) => {
+                            if let Some(drop_fn) = sparse_set.get_drop() {
+                                drop_fn(component_ptr);
+                            }
+                        }
+                    }
                 }
             }
             bundle_component += 1;

crates/bevy_ecs/src/storage/blob_vec.rs

@@ -366,6 +366,13 @@ impl BlobVec {
         unsafe { core::slice::from_raw_parts(self.data.as_ptr() as *const UnsafeCell<T>, self.len) }
     }
 
+    /// Returns the drop function for values stored in the vector,
+    /// or `None` if they don't need to be dropped.
+    #[inline]
+    pub fn get_drop(&self) -> Option<unsafe fn(OwningPtr<'_>)> {
+        self.drop
+    }
+
     /// Clears the vector, removing (and dropping) all values.
     ///
     /// Note that this method has no effect on the allocated capacity of the vector.

crates/bevy_ecs/src/storage/sparse_set.rs

@@ -300,6 +300,13 @@ impl ComponentSparseSet {
         })
     }
 
+    /// Returns the drop function for the component type stored in the sparse set,
+    /// or `None` if it doesn't need to be dropped.
+    #[inline]
+    pub fn get_drop(&self) -> Option<unsafe fn(OwningPtr<'_>)> {
+        self.dense.get_drop()
+    }
+
     /// Removes the `entity` from this sparse set and returns a pointer to the associated value (if
     /// it exists).
     #[must_use = "The returned pointer must be used to drop the removed component."]

crates/bevy_ecs/src/storage/table/column.rs

@@ -697,4 +697,11 @@ impl Column {
             changed_by.get_unchecked(row.as_usize())
         })
     }
+
+    /// Returns the drop function for elements of the column,
+    /// or `None` if they don't need to be dropped.
+    #[inline]
+    pub fn get_drop(&self) -> Option<unsafe fn(OwningPtr<'_>)> {
+        self.data.get_drop()
+    }
 }