Emit DeliveryResult in the access telemetry row #1155

Description

During PR #1074 review, a DeliveryOutcome.result / DeliveryResult value was proposed for classifying streamed delivery as complete, partial or error. The original report said this value was present but unused behind #[allow(dead_code)], and proposed a delivery_result low-cardinality field on access_logs_raw so truncated streams become visible in telemetry.

At checkout 666953a0d, a source search found no DeliveryResult, DeliveryOutcome or classify_stream_delivery symbol in the current core or Fastly adapter. The original implementation premise is therefore historical and must be revalidated before adding a telemetry field.

Proposed resolution

Confirm whether the delivery classification was removed, renamed or remains in another branch. If partial/errored streaming delivery is still not distinguishable in access telemetry and the product needs it, define an observable classification at the delivery boundary and carry it into the access row with a compatible schema migration. If the reviewed groundwork was removed and no consumer is planned, close this follow-up with that evidence.

Done when

• The current delivery and access-telemetry path is traced and the original premise is resolved.
• If the field is implemented, complete/partial/error cases are tested through emitted access rows, including compatible schema evolution.
• If no implementation is needed, the issue records why the unused groundwork and telemetry gap no longer apply.

Related

PR #1074 review; #1076 auction-timeline telemetry pattern.