SNOW-3190420: Add start_nbctl.sh validation check for Notebook custom…

[sfc-gh-jijiang]

Pre-review checklist

• I've confirmed that instructions included in README.md are still correct after my changes in the codebase.
• I've added or updated automated unit tests to verify correctness of my new code.
• I've added or updated integration tests to verify correctness of my new code.
• I've confirmed that my changes are working by executing CLI's commands manually on MacOS.
• I've confirmed that my changes are working by executing CLI's commands manually on Windows.
• I've confirmed that my changes are up-to-date with the target branch.
• I've described my changes in the release notes.
• I've described my changes in the section below.
• I've described my changes in the documentation.

Changes description

Note: This PR is stacked on top of #2809 image validation framework) which is still under review. Please treat this as an early preview — it is ready for feedback but should not be merged until the parent PR lands.

SNOW-3190420: Add start_nbctl.sh validation check for Notebook custom images

This PR adds a Notebook-specific validation check to the snow custom-image validate CLI command.

For Notebook vNext, PEP injects ./start_nbctl.sh /shared/nbctl as the container entrypoint args. If this script is missing or not executable in the custom image, the container will crash immediately on startup. This check catches that before deployment.

What changed:

• image_validation.yaml: Added required_scripts: [start_nbctl.sh] config entry
• manager.py: Added _check_required_scripts handler that runs docker run ... test -x <script> to verify each script exists and is executable
• test_manager.py: Added unit tests for pass (script present) and fail (script missing) scenarios, updated mock helper to support missing_scripts parameter

Validation:

• All 19 unit tests pass (python3 -m pytest tests/custom_images/ -v)
• Manually verified start_nbctl.sh exists and is executable in the official snowflake-notebook:ml-cpu image