The commit-time hold keeps the telemetry trio for its own attempt's
success emit. Clear it at the start of the next attempt (and keep
pool-saturation refusal emits from inheriting the last attempt's
payload) so a later emit cannot report the previous attempt's numbers.
Follow-up on the #118580 review.
[Ported onto main, where _emit_compression_attempt_telemetry now lives in
agent/conversation_compression_telemetry.py: the include_last_telemetry
parameter moved there with the same behavior.]
(cherry picked from commit 1c05c6a6e713cd77aee3111d5a809572ad3f7d0c)
(cherry picked from commit 5a551af25a643d0d6c1f1ded2ce9d9c4a52276ba)
5ce01755b5