fix: cleanup orphaned entities after episode deletion

[Parteeksachdeva]

Summary

This PR fixes an issue where orphaned Entity nodes were not cleaned up when deleting episodes via remove_episode().

Due to LLM non-determinism, Graphiti can sometimes create Entity nodes without corresponding MENTIONS relationships. These entities were invisible to the existing cleanup logic and accumulated indefinitely in the database.

Type of Change

• Bug fix
• New feature
• Performance improvement
• Documentation/Tests

Objective

Fix an issue where Entity nodes without MENTIONS relationships are not cleaned up when an episode is deleted.

Due to LLM non-determinism or partial ingestion failures, entities can be created without corresponding MENTIONS edges. The existing remove_episode() logic only considers entities reachable via MENTIONS, causing these orphaned entities to persist indefinitely and bloat the database.

This PR adds a post-deletion cleanup step that removes all Entity nodes in the same group_id that have zero MENTIONS relationships after an episode is deleted.

Testing

• Unit tests added/updated
• Integration tests added/updated
• All existing tests pass

Breaking Changes

• This PR contains breaking changes

No breaking changes. Behavior is strictly additive and limited to cleanup during episode deletion

Checklist

• Code follows project style guidelines (make lint passes)
• Self-review completed
• Documentation updated where necessary
• No secrets or sensitive information committed

Related Issues

Closes #1083

[danielchalef]

All contributors have signed the CLA ✍️ ✅
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[Parteeksachdeva]

I have read the CLA Document and I hereby sign the CLA