from dateutil import parser
import pytz

def validate_and_convert_date(date_input: str) -> str:
    """
    Converts a given date string to a standardized RFC 3339 format.

    This function attempts to parse a given date string into a `datetime` object using the `dateutil` parser. 
    If the input string includes timezone information, it converts the time to UTC. If no timezone is present, 
    it assumes UTC as the default timezone. The resulting date is then formatted into an RFC 3339-compliant 
    string, using ISO 8601 format with a 'Z' suffix for UTC.

    Args:
        date_input (str): A date string in a recognizable format. The input string must be parsable by the `dateutil` parser.

    Returns:
        str: A string representing the date in RFC 3339 format (e.g., 'YYYY-MM-DDTHH:MM:SSZ').

    Raises:
        ValueError: If the input date string is invalid or cannot be parsed into a `datetime` object.

    Notes:
        - RFC 3339 requires timestamps to use ISO 8601 format. If the input string's timezone is UTC, the result will use a 'Z' suffix.
        - If the input string lacks timezone information, UTC is assumed by default.
    """
    # TODO: Implement this function
    pass