feat: add custom_search_prompt for graph entity extraction

[pmukeshreddy]

Summary

Adds custom_search_prompt config option for customizing entity extraction during graph search.

Problem

The search path uses a hardcoded prompt in _retrieve_nodes_from_data(). This doesn't work for domain-specific use cases where:

• Entity types need to be constrained (e.g., only Medication, Condition)
• Terminology mapping is needed (e.g., "heart attack" → "Myocardial Infarction")

Solution

New config option custom_search_prompt parallel to existing custom_prompt (which is for add path).

Usage

config = {
    "graph_store": {
        "provider": "neo4j",
        "config": {...},
        "custom_search_prompt": "Extract medical entities: Medication, Condition, Allergy. Patient: {user_id}"
    }
}

Changes

• graphs/configs.py: Added custom_search_prompt field to GraphStoreConfig
• memory/graph_memory.py: Use custom prompt in Neo4j implementation
• memory/memgraph_memory.py: Use custom prompt in Memgraph implementation
• graphs/neptune/base.py: Use custom prompt in Neptune implementation

Fixes #3299

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