fix(JSONMissingBlock): allow app block references

[chippleh1392]

What are you adding in this PR?

Fixes a false positive where JSONMissingBlock reports app block references as missing local block files when the containing schema allows @app.

App block references use shopify://apps/... types and do not map to local blocks/*.liquid files. This change skips local file validation for those references while still requiring the containing section or block schema to include @app.

Regression coverage includes schemas allowing @app with @theme, schemas allowing only @app, and schemas that do not allow app blocks.

Fixes #1144

What's next? Any followup issues?

None planned.

Tophatting

Run Theme Check against a JSON template containing a shopify://apps/... block reference:

• With @app in the containing schema, JSONMissingBlock does not report an offense.
• Without @app, JSONMissingBlock reports that the block type is not allowed.
• Local block validation continues to report missing or disallowed local blocks.

Validation completed:

• pnpm build
• CI=true pnpm exec vitest run
• pnpm type-check
• pnpm format:check

Before you deploy

• I included a patch bump changeset

[chippleh1392]

I have signed the CLA!