Make AbstractBrowserlessExtension implement BrowserlessDsl

[mcollovati]

Part of #120. Blocked by #121.

AbstractBrowserlessExtension (the @RegisterExtension programmatic API) is a third, verbatim copy of the DSL (navigate ×4, find ×2, findInView, getCurrentView, roundTrip, fireShortcut). Its private getUI() already implements exactly the currentUI() contract (returns UI.getCurrent(), throws BrowserlessTestSetupException if null).

Proposal

Declare AbstractBrowserlessExtension implements BrowserlessDsl, expose getUI() as public UI currentUI(), and delete the duplicated DSL method bodies. (Keep runPendingSignalsTasks until #125.) Low risk — identical public signatures.

Acceptance criteria

• The extension declares no DSL bodies beyond currentUI() (and, for now, runPendingSignalsTasks).
• BrowserlessExtension-based and per-class-lifecycle tests pass unchanged.

Affected files

junit6/.../AbstractBrowserlessExtension.java.