Two hardening guards extracted from #82766 by @StanleyStetson:
- bool is an int subclass, so a JSON `true` in truncate_before_user_ordinal
coerced via int() to ordinal 1 and aimed a CONFIRMED rewind at the second
user turn — the same silent-loss class as #82756. Reject with 4004.
- confirm_truncate with no truncation target is leaked client rewind state
on an ordinary submit; fail fast with 4004 instead of silently ignoring
the flag, so the corrupted client state is surfaced.
Part of the composite fix for #82756.
StanleyStetson