Add CLI integration test for creating a site from a custom Blueprint

[gcsecsey]

Related issues

• Fixes STU-1871
• Stacked on Add CLI integration test for site create (custom name, domain, HTTPS) #3947

How AI was used in this PR

I used Claude to review the existing Playwright blueprint tests and the new CLI e2e harness from #3947, plan the migration, and to write the test. I reviewd and verified all changes.

Proposed Changes

Blueprint coverage currently lives only in apps/studio/e2e/blueprints.test.ts, which boots the entire desktop app through Playwright to create a site from a custom Blueprint and then inspects wp-admin — slow, flaky, and coupled to the UI. This continues the "Migrate Studio E2E tests to CLI" effort by exercising the same capability directly through the CLI.

• Adds a real integration test that runs studio site create --blueprint <file> against an isolated config directory and asserts on the result: a site is created from a custom Blueprint, and the Blueprint's install steps land their files on disk. Because the Blueprint is applied during create even with --no-start, the test can verify real effects (a theme and a plugin installed) without booting a server. It covers four step types from the original suite — install theme, install plugin, run PHP, and WP-CLI.
• Closes a gap in the Add CLI integration test for site create (custom name, domain, HTTPS) #3947 harness: the helper isolated the config directory but not the legacy Electron appdata directory, so the Studio-compatibility startup migration could find a real pre-split appdata-v1.json and abort the CLI on a developer's machine (it passed in CI only because CI starts clean). The harness now isolates that path too, which also fixes the sibling create.e2e.test.ts locally.

Per the issue, activation/option-state assertions (which require reading the site's SQLite database) and error/validation cases are left as follow-ups, and the Playwright blueprints.test.ts is kept for now.

Testing Instructions

1. Build the CLI: npm run cli:build.
2. Run the new suite (needs a network connection — the install steps download from wordpress.org):
   npm test -- apps/cli/commands/site/tests/create-blueprint.e2e.test.ts --tagsFilter='e2e'
   Confirm all four tests pass.
3. Confirm the fast suite is unaffected:
   npm test -- apps/cli/commands/site/tests/create-blueprint.e2e.test.ts --tagsFilter='!e2e'
   The four tests are skipped.
4. Regression check for the harness fix — confirm the sibling test also passes:
   npm test -- apps/cli/commands/site/tests/create.e2e.test.ts --tagsFilter='e2e'

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?