Validate model metadata within subgraphs; add iter_all_nodes helper

[benceruleanlu]

This pull request improves the validation script to correctly handle models referenced inside subgraphs and tightens model metadata checks.

Changes

• Add iter_all_nodes helper to return a flat list of all nodes, including those defined under definitions.subgraphs[].nodes.
• Use the helper to:
  • Suggest appropriate model loader nodes when a template incorrectly uses top-level models.
  • Validate that each model listed in a node's properties.models has a corresponding entry in that node's widgets_values.
• Improve type annotations and error messages for clarity.

Why
Some workflows place loader nodes inside subgraphs. Previously, validation only considered top-level nodes, which could miss these cases.

Notes

• Only scripts/validate_templates.py is modified.
• Local run confirmed no regressions; the validation script runs cleanly.

If you prefer a different branch name, title, or PR wording, I can update accordingly.

[benceruleanlu]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

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[christian-byrne]

Should we run it against the original bugged workflow to verify it works?

[comfyui-wiki]

Let me add a wrong file and test it.

[benceruleanlu]

Sorry, forgot to mention I did this, it seems to work well

Should we run it against the original bugged workflow to verify it works?