Self-review of the generation marker. MAX(ended_at) alone is wall-clock: an
NTP correction between two resets writes a SMALLER boundary, MAX keeps
returning the older one, and the next conversation silently reuses the
previous generation -- two conversations on one routing key, which is the
defect this PR exists to remove.
latest_conversation_boundary now returns (count, latest_ended_at) and the
marker is 'count:ended_at'. The two halves fail under different conditions --
a backwards clock defeats the timestamp, retention pruning of an old ended row
decrements the count -- so a generation repeats only if both happen at once.
The pair is deliberately biased toward changing: a spurious change costs one
cold prompt-cache bucket, a repeat would merge two conversations.
Pinned by test_a_backwards_clock_does_not_reuse_a_generation, which rewrites
the second boundary to land before the first and asserts three conversations
still resolve to three distinct scopes.
Refs #96811
joaomarcos
e5bce4df4b