Fix flaky OpenAI test: accept hex color with or without # prefix
OpenAI's structured output sometimes returns '0000FF' instead of '#0000FF'.
Both are valid hex color representations, so the test should accept either.
ci: skip quickstart notebook in notebook-checker workflow
The quickstart notebook requires an OpenAI API key which is not available
in CI. Skip it until we have a proper solution for testing notebooks with
API dependencies.