Connection node subnodes

[ivaylo-matov]

Add a targeted encoder for ConnectionNode-like objects to preserve callable .NET methods.

Python.NET was auto-encoding IEnumerable .NET objects (like ConnectionNode) into Python iterables, which stripped the original .NET instance and made methods like SubNodesOfSize or ExistingConnections unavailable. This change ensures these objects remain wrapped as CLR objects.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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