Workspace config: Wrong array concatenation in listAll() and getAllCollectionNames() of storage

Problem/Motivation

The methods WseConfigDatabaseStorage::listAll() and WseConfigDatabaseStorage::getAllCollectionNames() are joining the values of the inner storage with the workspace storage using += operator. This is wrong because the return values are numeric arrays and using += will overwrite existing keys read from the wse storage.

Steps to reproduce

On Vanilla Drupal 9:

1. Enable wse_config and asset_injector modules
2. Create 2 asset injector CSS configs in the live workspace
3. Switch to the stage workspace and create another asset injector CSS config
4. Navigate to /admin/config/development/asset-injector/css (still inside the stage workspace) and see only 2 configs instead of the expected 3 (2 of the live workspace + 1 created in the stage workspace)

Proposed resolution

Use array_merge() instead of the += operator to preserve keys when joining the lists.