drupal-contrib D12: core/tests missing because dev-branch dist zip strips it via .gitattributes export-ignore

[rfay]

Summary

drupal-contrib's D12 cell (drupal_version=12) installs drupal/core from the 12.x-dev branch (there's no stable D12 tag yet — see #212). Composer's default prefer-dist install for a -dev/branch version fetches a GitHub zipball (https://api.github.com/repos/drupal/core/zipball/<sha>), which is a git archive snapshot. git archive honors .gitattributes export-ignore rules, and drupal/core's own .gitattributes marks /tests (and other dev-only paths) export-ignore — so the dist zip silently omits core/tests entirely.

Any test that references core directly — including ddev phpunit web/core/modules/system/tests/src/Functional/System/PageTitleTest.php, the project-agnostic smoke test added to CI in #215 — fails with:

Cannot open bootstrap script "/var/www/html/web/core/tests/bootstrap.php"

CI run: https://github.com/ddev/coder-ddev/actions/runs/36280470996/job/108511195288 (Contrib skipto D12 (plain, GH)).

D10/D11 are unaffected — they resolve to real tagged releases, whose composer dist packages already include core/tests (drupal.org's release packaging isn't a plain git archive).

Root cause, verified

• vendor/composer/installed.json showed drupal/core's dist.url was a GitHub zipball, source.type was git.
• A plain git clone https://github.com/drupal/core.git (same ref) does include tests/ — confirming export-ignore is the mechanism, not a genuinely trimmed upstream branch.
• Forcing composer config preferred-install.drupal/core source before ddev poser runs (so Composer does a real git clone instead of downloading the zipball) resolves it: verified core/tests present and ddev phpunit --filter testTitle web/core/modules/system/tests/src/Functional/System/PageTitleTest.php passes.

Fix

Since this was found by (and blocks CI on) #215, the fix — set composer config preferred-install.drupal/core source only when drupal_version=12, in drupal-contrib/template.tf's Phase 5 composer.json setup — is included as an additional commit on that PR rather than a separate one, to keep its new CI coverage green. Filing this issue for traceability and in case the same class of problem needs to be considered again (e.g. if drupal-core's own D12 handling — it doesn't currently support an unreleased major the way drupal-contrib does — ever needs the same treatment).