[DB] Real-driver nested-module test

[cssbruno]

Add a real-driver nested-module test for the DB addon.

Scope:

• Exercise DB addon helpers with a real database driver in an optional/nested module.
• Keep third-party database driver dependencies out of the root module graph.

Acceptance criteria:

• Nested module test runs independently.
• Root module dependencies remain unchanged.
• CI or docs identify how to run the real-driver test.