[FIX] utils: Correct validation of mount target in shadow DOM/iframe

[abz-odoo]

Previously, validateTarget only checked if the target element or its host (if it was a ShadowRoot) was directly contained in the document body. This failed in cases where the target element was nested inside a shadow DOM, which itself was attached to the document.

This commit introduces a new helper isAttachedToDocument that traverses through parent nodes and shadow roots to ensure that the target is ultimately attached to the given document. Additionally, it now throws a clear error if document.defaultView is missing, indicating that the target document is detached or invalid.

This ensures proper validation of mount targets, including complex scenarios with shadow roots and iframes.

Before this commit, this threw an OwlError

const shadow = fixture.attachShadow({ mode: "open" });
const shadowDiv = document.createElement("div");
shadow.append(shadowDiv);
const app = new App(SomeComponent);
const comp = await app.mount(shadowDiv);

Now this is a valide case