Editing the only turn of a thread reran the original prompt: the model
answered the question the edit was replacing while the UI showed the
edited text, and the edit vanished on reload.
The replay-base lookup decided whether a checkpoint predates the target
user message by message id alone. DynamicContextMiddleware re-keys the
first user turn to `{id}__user` mid-run, so every checkpoint written
before it holds the same prompt under an id the lookup cannot match. The
scan walked past those and anchored inside the run that produced the
turn — a checkpoint that still contains the original prompt and owns the
injection node's pending writes, which the replay then re-added after the
edited message.
Require the replay base to be a settled checkpoint (no pending tasks) in
both the lineage walk and the chronological fallback. That rule is
middleware agnostic: the first turn now anchors on the thread's empty
initial checkpoint and later turns on the previous run's tail, which also
drops the existing reliance on LangGraph discarding a stale `__start__`
write.
Edit replay additionally passes `head_checkpoint` so it resolves its base
lineage-first like regenerate does, and a replayed user message is
restored to its pre-swap id: replaying `{id}__user` into a state that has
no reminder yet makes the middleware treat the turn as already injected
and silently drops its date and memory block.
Frontend: a prepared replay masks the turn it supersedes, so the
optimistic-message baseline is taken from the post-mask human count. The
pre-mask count can never be exceeded when the replay puts exactly one
human message back, and on the first turn the runtime re-keys the
replacement message so identity comparison cannot stand in for the count.
Fixes#4531
