fix(heal): demote per-object logs and cap erasure-set failure warns
Follow-up to rustfs/rustfs#5716. Per-object heal task kinds (Object/Metadata/MRF/ECDecode) queued by MRF/autoheal/scanner loops emitted info!/warn!/error! lines per object: task lifecycle (started/completed/timed_out/failed), the missing-object warn, queue admission full/drop/displacement warns, retry-admission decisions, and uncapped per-object warns in erasure-set sweeps.
Add a shared demote_to_debug_when! macro that keeps aggregate task kinds and admin/internal requests at operator-visible levels while demoting per-object occurrences to debug!, sample-cap the erasure-set transient_skip/failed warns per bucket via take_failure_log_sample (reusing the heal_bucket_objects precedent), demote the per-retry admission decision logs to debug! (covered by rustfs_heal_admission_total and the scheduler task_retrying/task_failed events), and record the previously unmetered duplicate-admission outcome.
Extend scripts/check_logging_guardrails.sh with injection-verified regression guards and run it in both quick-checks jobs (ci.yml and its ci-docs-only.yml mirror).
