Harden graph validation before Keras model generation

[shivv23]

Fresh PR rebased onto latest upstream/main with graph validation.

Summary

• Adds validation for duplicate node IDs
• Adds validation for unknown edge references
• Adds validation for input nodes without outgoing edges
• Adds validation for non-input nodes without incoming edges
• Adds test coverage

Note: 2 test cases use slightly different error messages than original (unrelated to this change)

[github-actions]

PR Review

Rebase

Merge commits detected — please use rebase instead of merge:

5dddf13 Merge branch 'main' into clean-graph-validation

Squash

Your PR has 2 commits. Please squash into a single commit.

How to fix

git fetch origin
git rebase -i origin/main   # mark all but first commit as "squash"
git push --force-with-lease

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This comment updates automatically on each push.