flexible backend support by adding an abstraction layer (part of #128)

[Srishti-1806]

Describe your changes

(#128 contains the full proposed pipeline from the GSoC proposal. This PR is part of breaking that contribution into smaller, focused PRs for better readability and review.)

This PR addresses a bug in KD-Tree based node mapping when connecting nodes across graphs.

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Issue

Incorrect node mappings were observed during KD-Tree neighbor queries in cross-graph connections.
This led to inconsistent or incorrect graph connectivity.

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Fix

• Corrected index handling during KD-Tree neighbor queries
• Ensured proper mapping between source and target graph nodes
• Validated consistency of cross-graph connections

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Result

• Accurate node mapping across graphs
• Stable and correct graph construction behavior
• Fixes the issue reported in [Bug] Wrong KDTree Node Mapping in connect_nodes_across_graphs #82

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Issue Link

Part of #128

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Type of change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📖 Documentation

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Testing

• ✅ Verified correct node mapping across graphs
• ✅ No incorrect neighbor assignments observed
• ✅ Existing tests pass

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Checklist before requesting a review

• My branch is up-to-date with the target branch
• Performed self-review
• Added necessary comments for clarity
• Ensured compatibility with existing code

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Additional Notes for Reviewers

This PR is part of a larger effort (#128) to modularize the following proposed features:

1. Backend Abstraction Layer ✅ (PR 1 - Done)
2. Spatial Indexing Optimization (KD-Tree / Ball Tree)
3. Temporal Graph Support
4. Configuration-Driven Graph Pipeline
5. Feature Engineering Layer
6. ML Pipeline Integration
7. Prebuilt Graph Architectures
8. Dependency & Packaging Enhancements

The original PR (#128) is kept open as a reference for the complete integrated pipeline.

[Srishti-1806]

##Screenshots proving successful implementation.