[WIP] Fix DiscreteNode type unhashable error when whitelisting

[Copilot]

• Inspect existing structure-learning whitelist handling and reproduce the unhashable DiscreteNode error locally
• Identify minimal code change to normalize whitelist/init_edges types for K2 learning
• Add focused regression test covering whitelist with string node names
• Run relevant tests before and after change (targeted first)
• Run code review and CodeQL checks, address findings if any
• Final verification and progress update