fix(screencast): register AnnotatePosition enum serializer

[Develop-KIM]

Summary

• AnnotatePosition had no Gson serializer registered, so page.screencast().showActions(new Screencast.ShowActionsOptions().setPosition(AnnotatePosition.TOP_RIGHT)) sent the raw constant TOP_RIGHT to the driver, which rejected it: position: expected one of (top-left|top|top-right|...).
• Registered it with ToLowerCaseAndDashSerializer (as used for ColorScheme, ServiceWorkerPolicy, …) so values serialize as top-right etc.

Fixes #1912

[Develop-KIM]

@microsoft-github-policy-service agree

[yury-s]

The change looks good, let's update the test

[yury-s]

let's turn it into and e2e test and check that actual playwright api call doesn't throw page.screencast().showActions(new Screencast.ShowActionsOptions().setPosition(AnnotatePosition.TOP_RIGHT))

[Develop-KIM]

Done — dropped the serializer unit test and moved the coverage into TestScreencast, going through the real API:

for (AnnotatePosition position : AnnotatePosition.values()) {
  AutoCloseable disposable = page.screencast().showActions(
    new Screencast.ShowActionsOptions().setPosition(position));
  assertNotNull(disposable);
  disposable.close();
}

Ran it against a real browser both ways. Without the serializer registration every position fails with position: expected one of (top-left|top|top-right|bottom-left|bottom|bottom-right), which is the error from #1912; with it the test passes.

I looped over all six values rather than just TOP_RIGHT so a future enum constant that doesn't round-trip gets caught too — happy to narrow it if you'd rather keep the test minimal.