Fix observers' access to relationship data during despawn_related

[muddxyii]

Objective

• Fix issue where observers cannot access relationship data during despawn_related
• Fixes despawn_related removes relationship before calling observers and hooks #20106

Solution

• Changed despawn_related to use get() instead of take() to preserve relationship components during the despawn process
• Collect entities into a vector before despawning to ensure the relationship data remains accessible to observers and hooks

Testing

• Added test despawn_related_observers_can_access_relationship_data that reproduces the issue scenario

[alice-i-cecile]

I've left a suggestion for a clarifying comment, but this is fundamentally a good fix and I really appreciate the regression test.