feat: add error handling for missing nodes and edges, introduce new API endpoints, and update ZepGraphiti class

[paul-paliychuk]

:rocket:	This description was created by Ellipsis for commit b454f87521a5bca7d8ff591e3e971d1e44dac888

Summary:

This PR adds error handling for missing nodes and edges, new API endpoints, and updates the ZepGraphiti class for these operations.

Key points:

• Adds error handling for missing nodes and edges.
• Introduces new API endpoints for managing nodes and edges.
• Updates ZepGraphiti class to support operations with error handling.
• Adds EdgeNotFoundError and NodeNotFoundError in graphiti_core/errors.py.
• Updates graphiti_core/edges.py and graphiti_core/nodes.py to raise errors.
• Adds API endpoints in server/graph_service/routers/ingest.py and server/graph_service/routers/retrieve.py.
• Updates ZepGraphiti class with methods get_entity_edge, delete_entity_edge, and delete_episodic_node.

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