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.
Achraf (abz)