Regression in 5.1.5: Secondary (CiviCRM) database source fails with "Cannot export database because the file system service is missing

After updating from 5.1.4 to 5.1.5 via Composer, backups using a secondary database source (CiviCRM) fail with:

Cannot export database because the file system service is missing.

The standard Drupal database source continues to work; only the secondary source fails. The failure is identical via the UI (Quick Backup → download) and via Drush (drush backup_migrate:quick_backup -vvv), ruling out destination or directory-permission causes. Nothing is written to watchdog; the error appears only in the backup result message.

Downgrading only backup_migrate from 5.1.5 to 5.1.4, with no other module changes, resolves it completely. This isolates the regression to a 5.1.5 change in how backup sources obtain the file_system service — the CiviCRM source's dependency appears to resolve to null at runtime.

Steps to reproduce:

Site with a secondary (CiviCRM) database source configured in Backup and Migrate.
On 5.1.4, run a quick backup of a secondary database (CiviCRM) → succeeds.
Update to 5.1.5, drush cr.
Run the same quick backup → fails with the error above.
Downgrade to 5.1.4, drush cr → succeeds again.

Environment:

Drupal 11.3.16
PHP 8.3.33
backup_migrate 5.1.5 (broken) / 5.1.4 (working)
CiviCRM 6.17.2, civicrm_entity 4.0.8
MySQL, Drush 13.7.6

Note: The CiviCRM source is provided by backup_migrate itself (confirmed by grep of the module directory), so the fix belongs in this project.

Second note: Help from Claude with writing this bug report.