Rebased onto main, where params are validated against tui_gateway/contracts
and unknown keys are rejected: the copy_parent_history / omit_messages flags
on session.create and session.branch now answered 4000. Keep both methods'
wire shape unchanged and give the two new methods their own contracts
(session.branch_stored, session.branch_whole) with messages_omitted results;
the handlers take the behaviour as keyword arguments, not wire flags.
ecacf3d0c9