[Themes] Verify existence of required theme files before initializing storefront session

[jamesmengo]

WHY are these changes introduced?

Adds validation to ensure required theme files exist before initializing the dev server session. This helps catch issues with incomplete or corrupted themes that could cause the dev server to fail when initializing the preview session.

• [Bug]: Shopify-cli 3.69.2. Your development session could not be created because the "_shopify_essential" could not be defined. Please, check your internet connection. #4735
• https://github.com/Shopify/develop-advanced-edits/issues/406

WHAT is this pull request doing?

• Adds a new verifyRequiredFilesExist function that checks for essential theme files (layout/theme.liquid and config/settings_schema.json)

How to test your changes?

1. Create a theme missing either layout/theme.liquid or config/settings_schema.json on your shop (more instructions below if needed)
2. Run the theme dev with the theme flag, and specify the --theme flag to point at the theme created above. Verify that it fails with appropriate error message

Creating Theme with Missing Assets

• Cherrypick this change on top of the PR branch
• p build
• Delete your development theme shopify-dev theme delete -d
• Run shopify-dev theme dev

The above command should create a new development theme with missing files - you should then see the error being rendered.

Measuring impact

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[jamesmengo]

• [Themes] Verify existence of required theme files before initializing storefront session #4810 👈
• main

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[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	72.04% (+0% 🔼)	8348/11588
🟡	Branches	68.48% (+0.01% 🔼)	4036/5894
🟡	Functions	71.32% (-0.01% 🔻)	2191/3072
🟡	Lines	72.42% (-0% 🔻)	7894/10901

Test suite run success

1890 tests passing in 865 suites.

Report generated by 🧪jest coverage report action from 2e91ff9

[jamesmengo]

/snapit

[github-actions]

🫰✨ Thanks @jamesmengo! Your snapshot has been published to npm.

Test the snapshot by intalling your package globally:

pnpm i -g @shopify/cli@0.0.0-snapshot-20241107193203

After installing, validate the version by running just shopify in your terminal
If the versions don't match, you might have multiple global instances installed.
Use which shopify to find out which one you are running and uninstall it.

[jamesmengo]

pivot: only show this when essential is missing to better determine the cause

[frandiox]

Looks good, hoping we can get more insights about the missing cookies with this enhanced error 🙌

[karreiro]

Thank you, @jamesmengo! I didn't have the chance to test this one, but these changes LGTM :)