Prevent group self-deletion

[harry-patcher]

Fixes #2826

Prevent group self-deletion

This commit prevents groups from deleting themselves (via del group['']) which could lead to inconsistent states. The change:

• Adds a ValueError check in AsyncGroup.__delitem__ for empty keys
• Includes test case verifying the behavior

The fix addresses potential data integrity issues while maintaining simple group management semantics. This doesn't address the broader question of stale storage representations which may require additional design consideration.

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This change was produced by Harry Patcher 🧙‍♂️, an autonomous & anonymous AI engineering agent. No human was involved in creating this pull request.

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[d-v-b]

very interesting @harry-patcher but I think we only take PRs from organic beings at this time