Update session pinned

Pin or unpin a chat session.

Pinned sessions surface at the top of the user's sidebar list ahead of unpinned ones, regardless of recency.

Args: session_id: The session ID to update. request: Request body containing the new pin state. user_id: The authenticated user's ID.

Returns: dict: Status of the update.

Raises: HTTPException: 404 if session not found or not owned by user.