feat(theme-check-common): warn when select default isn't a valid option

[SinhSinhAn]

What are you adding in this PR?

This adds a new theme-check, ValidSelectDefault, that warns when a select or radio setting's default value isn't one of the values declared in its options. Today you can ship a schema whose default doesn't exist in options and theme-check stays silent. The merchant then sees the editor silently fall back to the first option (or no selection), which is confusing and easy to miss in review.

solves #943

The check validates three places:

• A setting's default value.
• A preset's settings values (presets[i].settings).
• A section's default.settings values.

It runs in two variants, mirroring ValidVisibleIf:

• ValidSelectDefault (LiquidCheck) → {% schema %} in sections/*.liquid and blocks/*.liquid.
• ValidSelectDefaultSettingsSchema (JSONCheck) → config/settings_schema.json.

Severity is WARNING and it's recommended: true, so it surfaces out of the box without blocking themes that have legacy schemas.

Example

{
  "type": "select",
  "id": "alignment",
  "default": "this-is-not-an-option",
  "options": [
    { "value": "flex-start", "label": "t:options.alignment.left" },
    { "value": "center",     "label": "t:options.alignment.center" },
    { "value": "flex-end",   "label": "t:options.alignment.right" }
  ]
}

Now warns: Default value "this-is-not-an-option" for setting "alignment" is not one of the valid options: "flex-start", "center", "flex-end".

The preset case from the issue gets a warning pointing right at the offending value inside presets[i].settings.

What's next? Any followup issues?

None planned. The check leaves non-scalar mismatches to JSON schema validation, so it stays focused on the select/radio option case.

What did you learn?

The factory configs (all.yml / recommended.yml) are generated from meta.code, so two checks sharing the same code (the Liquid + JSON variants, like ValidVisibleIf) collapse to a single config entry.

Tophatting

• I added screenshots of the changes (before and after the changes if applicable)

Tested via unit tests rather than screenshots (this is a non-visual lint check). 13 new tests in valid-select-default/index.spec.ts cover:

• valid + invalid default for select and radio
• setting with no default (no-op)
• non-choice settings left alone
• valid + invalid preset values
• preset referencing a non-select setting (no-op)
• section default.settings invalid value
• config/settings_schema.json via the JSONCheck variant (valid, invalid, wrong-file path)
• combined default + preset case reporting both

✓ packages/theme-check-common/src/checks/valid-select-default/index.spec.ts (13 tests)

The full theme-check-common checks suite stays green (780 tests across 79 files), and tsc --noEmit passes.

Before you deploy

• This PR includes a new check

  • I included a minor bump changeset
  • I ran pnpm build and committed the updated configuration files (all.yml, recommended.yml)
    • If applicable, I've updated the theme-app-extension.yml config — not applicable; the check is generally relevant and isn't theme-app-extension specific.
    • [Shopifolk] I've made a PR to update the shopify.dev theme check docs — N/A (not a Shopifolk).

• My feature is backward compatible

[aswamy]

Hey @SinhSinhAn, thanks for another PR. Please ensure all of your PRs (including this one) follows our PR template to ensure everything is tested properly before submitting it:
https://github.com/Shopify/theme-tools/blob/main/.github/PULL_REQUEST_TEMPLATE.md

I've also written some review comments below.

[SinhSinhAn]

Thanks for the review @aswamy! Addressed everything:

• PR template - rewrote the description to follow the template (What/What's next/What did you learn/Tophatting/Before you deploy), with the new-check checklist filled in.
• Recommended check configs —-rebased onto main (so I'm on pnpm now), ran pnpm build, and committed the regenerated all.yml and recommended.yml. Both now carry a ValidSelectDefault entry at severity 1.
• Formatting - ran the formatter; the check files and checks/index.ts are prettier-clean.

The branch is rebased to a single commit on top of latest main. theme-check-common checks suite is green (780 tests) and tsc --noEmit passes.

[aswamy]

@SinhSinhAn please include a screenshot to ensure this VSCode extension was actually tested when running theme-tools. Not just unit tests.