Fix KDTree node mapping order in connect_nodes_across_graphs(#82)

[Ashu-Rajput08]

Describe your changes

This PR fixes a bug in connect_nodes_across_graphs where KDTree neighbour indices could be mapped to incorrect source nodes due to inconsistent node ordering.

Previously, the KDTree was constructed using the insertion order of G_source.nodes, while neighbour indices were later resolved using sorted(G_source.nodes). Since KDTree indices are positional, this mismatch could silently produce incorrect edge assignments when the insertion order differed from the sorted order.

The fix ensures that a single consistent source_nodes_list is used both when constructing the KDTree input (xy_source) and when mapping KDTree neighbour indices back to node labels.

Additionally, a regression test has been added that inserts nodes in a non-sorted order and verifies that the resulting edges match the expected nearest-neighbour mapping.

No new dependencies are required for this change.

Issue Link

Closes #82

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

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