fix: repair notebook CI (dead model, missing API key, pyarrow type bug)

[andreatgretel]

Summary

• Replace dead meta/llama-4-scout-17b-16e-instruct model with nvidia/nemotron-nano-12b-v2-vl in tutorial 4 (model returns 404 on inference despite still being listed in the NVIDIA API catalog)
• Add OPENROUTER_API_KEY to build-notebooks.yml so tutorials 5/6 can authenticate with OpenRouter (missing since feat: add image generation support with multi-modal context #317/docs: add image generation documentation and image-to-image editing tutorial #319 added these notebooks). Also fixes build-docs and pack-tutorials which call the same reusable workflow.
• Fix isinstance(x, list) → isinstance(x, str) in tutorial 6's display_image — parquet-loaded values are pyarrow ListScalar, not Python lists (introduced in feat: auto-detect ImageContext format for image-to-image generation #342)
• Regenerate Colab notebooks

Test plan

• CI Build notebooks workflow passes: https://github.com/NVIDIA-NeMo/DataDesigner/actions/runs/22309476905

[greptile-apps]

Greptile Summary

Fixes three CI failures in notebook workflows: replaces a dead vision model, adds missing API key authentication, and fixes a type-checking bug for pyarrow-loaded data.

• Replaced meta/llama-4-scout-17b-16e-instruct (returns 404) with nvidia/nemotron-nano-12b-v2-vl in tutorial 4
• Added OPENROUTER_API_KEY environment variable to CI workflow (missing since tutorials 5/6 were added in feat: add image generation support with multi-modal context #317/docs: add image generation documentation and image-to-image editing tutorial #319)
• Fixed display_image function in tutorial 6: changed isinstance(x, list) → isinstance(x, str) to handle pyarrow ListScalar types from parquet files (bug introduced in feat: auto-detect ImageContext format for image-to-image generation #342)
• Regenerated all Colab notebooks with these fixes

Confidence Score: 5/5

• Safe to merge - fixes critical CI failures with targeted, well-tested changes
• All three fixes address real production issues with minimal scope. Model replacement fixes 404 errors, API key addition enables authentication, and type check fix handles pyarrow types correctly. CI workflow already passed validating the fixes.
• No files require special attention

Important Files Changed

Filename	Overview
.github/workflows/build-notebooks.yml	Added OPENROUTER_API_KEY env var to fix authentication for tutorials 5/6
docs/notebook_source/4-providing-images-as-context.py	Replaced dead model meta/llama-4-scout-17b-16e-instruct with nvidia/nemotron-nano-12b-v2-vl
docs/notebook_source/6-editing-images-with-image-context.py	Fixed display_image bug to handle pyarrow ListScalar values from parquet files

_{Last reviewed commit: 103799b}