After context compression, the preserved todo list was unconditionally
appended as a standalone user message. When the compressed transcript
already ends with a user message (common case), this creates consecutive
user/user turns — a role-alternation violation some providers reject.
Fix: fold the snapshot into the trailing user message (blank-line separated)
when one exists with plain-string content. Falls back to append when the
tail is non-user, empty, or has structured (list) content.
Rebased on current upstream/main.
Closes#53890
d2bb6cc251