fix(check-extension): mirror path repos to prevent symlink errors with template files (#218)

Composer installs path repositories as symlinks by default, which causes Magento's template engine to fail when resolving .phtml files. Setting `COMPOSER_MIRROR_PATH_REPOS=1` on all composer install steps forces a copy instead, matching how the package would be installed from Packagist in production.

Adds a .phtml template and an integration test to the demo package that
renders it via Magento's template engine. Without COMPOSER_MIRROR_PATH_REPOS=1
the path repo is installed as a symlink and the test fails; with mirroring
it passes.

Closes #217

.github/workflows/check-extension.yaml

@@ -75,6 +75,7 @@ jobs:
           run: composer install
           env:
             COMPOSER_AUTH: ${{ secrets.composer_auth }}
+            COMPOSER_MIRROR_PATH_REPOS: 1
 
         - name: Configure phpunit.xml.dist
           working-directory: ${{ steps.setup-magento.outputs.path }}
@@ -136,6 +137,7 @@ jobs:
           run: composer install
           env:
             COMPOSER_AUTH: ${{ secrets.composer_auth }}
+            COMPOSER_MIRROR_PATH_REPOS: 1
 
         - name: Enable all modules
           working-directory: ${{ steps.setup-magento.outputs.path }}
@@ -205,6 +207,7 @@ jobs:
         run: composer install
         env:
           COMPOSER_AUTH: ${{ secrets.composer_auth }}
+          COMPOSER_MIRROR_PATH_REPOS: 1
 
       - uses: graycoreio/github-actions-magento2/get-magento-version@main
         id: magento-version