fix: traverse shadow roots in toHaveFocus

[SAY-5]

What:

Traverse shadow roots when resolving the active element so toHaveFocus matches elements focused inside open shadow DOMs.

Why:

document.activeElement returns the shadow host rather than the actually focused descendant, so toHaveFocus could never pass for elements inside a shadow root. Closes #722.

How:

Added a getDeepActiveElement helper that walks shadowRoot.activeElement chains, and use it in both the pass check and the received-element message. Tests cover a single shadow root and a nested shadow root.

Checklist:

• Documentation N/A
• Tests
• Updated Type Definitions N/A
• Ready to be merged