Fix missing default None for add_episode_bulk

[danielchalef]

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:rocket:	This description was created by Ellipsis for commit 591fc49ea484e55348331f5b2c804c6221d385a1

Summary:

Adds default None to group_id parameter in add_episode_bulk method in graphiti_core/graphiti.py for consistency.

Key points:

• Adds default None to group_id parameter in add_episode_bulk method in graphiti_core/graphiti.py.
• Ensures consistency with add_episode method which already has group_id: str | None = None.

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