Previously the deep_link parameter only had two effective states:
False (default) accepted any /start command regardless of arguments,
and True required arguments to be present.
Change the default to None (accept both, backward compatible) so that
False can mean "reject if deep-link arguments are present", which is
the intuitive expectation when explicitly passing deep_link=False.
Closes#1713
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
