Unify test(Class, Y) tester-resolution semantics across DSL implementations

Part of #120. Blocked by #121.

test(Class<T> tester, Y component) behaves differently per entry point:

• BaseBrowserlessTest → TesterRegistry.instantiate(tester, component) — no resolution, instantiates exactly the requested tester.
• BrowserlessUIContext → internalWrap(tester, component) — prefers a more specific registered tester (@Tests resolution).

So test(SomeTester.class, component) can return different types depending on the entry point.

Proposal

Decide the intended contract (likely the resolving internalWrap behavior, consistent with single-arg test(Y)), unify both, then move the method into BrowserlessDsl as a default.

Note: behavior decision + likely bug fix; needs a regression test (TesterResolutionTest).

Acceptance criteria

• test(Class, Y) has one documented behavior across all implementations.
• It is a default on BrowserlessDsl; per-class copies removed.
• Resolution behavior covered by a test.

Affected files

shared/.../BrowserlessDsl.java, shared/.../BaseBrowserlessTest.java, shared/.../BrowserlessUIContext.java, junit6/.../TesterResolutionTest.java.

Start with shared/.../BrowserlessDsl.java, then compare test(Class, Y) in BaseBrowserlessTest.java and BrowserlessUIContext.java. Read junit6/.../TesterResolutionTest.java and add regression coverage for the chosen resolution behavior. Done means one documented behavior, a default method on BrowserlessDsl, and no per-class copies.