fix: PHPStan and test strictness improvements

Add bin/ scripts to PHPStan scanFiles, suppress property.onlyWritten for
by-ref test properties, add exhaustive default match arm, use explicit
expects() on mock stubs, fix data provider return type, and simplify
redundant null check in NotificationServiceTest.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

phpstan.neon

@@ -3,6 +3,15 @@ parameters:
     phpVersion: 80499
     scanFiles:
         - web/index.php
+        - bin/cli-bootstrap.php
+        - bin/module-cli-bootstrap.php
+        - bin/module-migrate.php
+        - bin/module-deactivate.php
+        - bin/module-build.php
+        - bin/module-assets-sync.php
+        - bin/module-permissions-sync.php
+        - bin/module-runtime-sync.php
+        - bin/doctor.php
     paths:
         - config
         - lib