Extract the browserless DSL into a BrowserlessDsl interface mixin

Part of #120.

The DSL methods (navigate, find, findInView, getCurrentView, fireShortcut, roundTrip, test) are public instance methods on BaseBrowserlessTest, so a test can only get them by extending that class. Composition-style tests (already extending another base, or wanting the DSL on a helper) cannot. The same DSL is duplicated in BrowserlessUIContext.

Proposal

Introduce a public interface BrowserlessDsl (in com.vaadin.browserless, alongside TesterWrappers / Locators) with:

• a single SPI hook UI currentUI() — implementors return the UI to operate on, performing any activation/validation;
• default methods delegating to the static helper: navigate (×4), find (×2), findInView, getCurrentView, fireShortcut, roundTrip, and the single-arg test(Y).

Supporting changes:

• Rename the internal static helper BrowserlessDSL → BrowserlessDslImpl (it is package-private / internal-only).
• BaseBrowserlessTest implements BrowserlessDsl with currentUI() → existing verifyAndGetUI(); drop the moved bodies. roundTrip changes from protected static to the inherited public default (existing unqualified callers still compile; ComponentTester.roundTrip() calls the helper directly).
• BrowserlessUIContext implements BrowserlessDsl with currentUI() → { activate(); return ui; }; drop the moved bodies.
• Add a test proving the mixin works on a class that does not extend BaseBrowserlessTest, and document the currentUI() contract + a raw-class usage example in the interface javadoc.

Out of scope (separate issues)

runPendingSignalsTasks (state-bound) → #125, test(Class, Y) (divergent semantics) → #124, and AbstractBrowserlessExtension migration → #123. Tracked under #120.

Non-breaking

Moved methods keep identical public signatures; subclass overrides still win over defaults; super.find(...) resolves through the inherited default. testingEngine() stays protected (not on the interface). Only residual: roundTrip protected static → public instance default.

Reference: proof-of-concept on branch refactor/browserless-dsl-mixin (commit 28c2818).

Acceptance criteria

• BrowserlessDsl exists with the currentUI() hook + listed defaults.
• BaseBrowserlessTest and BrowserlessUIContext implement it and no longer declare the moved methods.
• A raw-class (non-BaseBrowserlessTest) test exercises the DSL via the mixin.
• Full suite passes: mvn -o -am -pl shared,junit6,spring,quarkus test.

Affected files

New shared/.../BrowserlessDsl.java; rename shared/.../BrowserlessDSL.java → BrowserlessDslImpl.java; shared/.../BaseBrowserlessTest.java, shared/.../BrowserlessUIContext.java, shared/.../ComponentTester.java; new mixin test under junit6/src/test.

Start with the affected shared files, especially BaseBrowserlessTest.java, BrowserlessUIContext.java, BrowserlessDSL.java, and ComponentTester.java, then review the proof-of-concept branch for the intended interface shape. Add the raw-class mixin test under junit6/src/test and run mvn -o -am -pl shared,junit6,spring,quarkus test. Done means the listed acceptance criteria pass without breaking existing public signatures.