Plugin Builder local tests already provide Postgres and Azure Blob through PluginBuilder.Tests/docker-compose.yml, but email testing currently depends on an external Mailpit instance from another BTCPay test stack.
We should add Mailpit directly to the Plugin Builder test compose, following the same pattern used in BTCPay’s test setup.

This helps test verification emails, password reset emails, and SMTP failure/retry flows without relying on unrelated running containers.