[gitlab-runner] docker-autoscaler prunes active instances mid-job on v19.3.1, independent of token rotation

## Observed symptom docker-autoscaler prunes active instances mid-job on v19.3.1, independent of token rotation A user on GitLab Runner v19.3.1 with the AWS fleeting plugin reports docker-autoscaler pruning active instances while jobs are still running, producing SSH tunnel EOF errors and job system failures. This looks like the same symptom as https://gitlab.com/gitlab-org/gitlab-runner/-/issues/39380, but the known causes there don't apply: - Token expiration was already disabled (rules out the token-rotation pruning bug fixed by https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/6810). - No older Runner versions are active in the fleet (rules out a second logical runner from a stale version). - Only on-demand EC2 instances are in use, no spot instances (rules out spot-interruption pruning). ## Literal error text ``` error during connect: Get "http://internal.tunnel.invalid/v1.55/containers/<redacted>/json": ssh tunnel: EOF (%!s(<nil>)) (docker_command.go:151:0s) ``` ## Environment GitHub gitlab-org/gitlab-runner #39811 ## Trigger / context Public GitHub report in gitlab-org/gitlab-runner (FRESH_30D). ## Source evidence - https://gitlab.com/gitlab-org/gitlab-runner/-/issues/39811 (2026-09-14T10:00:54.970Z) — evidence_class: PRIMARY_ISSUE_REPORT — #39811 ## Freshness - classification: FRESH_30D - retrieved_at: 2026-10-02T10:50:53.328362Z - scout_id: C2bf1742b21 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.