refactor(agent): Trim sysfs-first cleanup nits

- Extend changes/11223.enhance.md with a short note that block-I/O
  readings may also step down on cgroup v1 hosts (the sysfs path reads
  blkio.throttle.io_service_bytes; the API path reads
  io_service_bytes_recursive).
- Drop TestWarnCgroupFallbackOnce::test_evicts_beyond_limit — it was
  re-verifying cachetools.LRUCache eviction, not project logic.
  test_deduplicates_per_container already covers the project contract.

Refs #11220
Refs #11223

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: rapsealk

changes/11223.enhance.md

@@ -1,3 +1,4 @@
 Default to cgroup (sysfs) stat collection on native Linux hosts, falling back to the Docker API only on linuxkit or read failure.
 Note: reported container memory usage may step down on hosts previously using `stats-type: docker`, because sysfs excludes inactive file cache (matching `docker stats`).
-Dashboards or autoscaling thresholds tuned to the old (higher) values should be re-evaluated after upgrade.
+Block-I/O readings may also shift on cgroup v1 hosts, because the sysfs path reads `blkio.throttle.io_service_bytes` while the Docker API path reads `blkio_stats.io_service_bytes_recursive` (which sums across nested cgroups).
+Dashboards or autoscaling thresholds tuned to the old values should be re-evaluated after upgrade.

tests/unit/agent/test_docker_intrinsic.py

@@ -352,28 +352,6 @@ def test_deduplicates_per_container(
         warn_records = [r for r in caplog.records if r.levelname == "WARNING"]
         assert len(warn_records) == 1
 
-    def test_evicts_beyond_limit(
-        self,
-        caplog: pytest.LogCaptureFixture,
-    ) -> None:
-        """When the bounded cache overflows, the oldest entry is evicted and a
-        previously-seen container may warn again."""
-        cap = intrinsic._CGROUP_FALLBACK_WARN_CACHE_SIZE
-        first_cid = "first_container"
-
-        with caplog.at_level("WARNING", logger="ai.backend.agent.docker.intrinsic"):
-            # First warn for `first_cid`.
-            _warn_cgroup_fallback_once("CPUPlugin", first_cid)
-            # Fill the cache with `cap` distinct new entries to evict `first_cid`.
-            for i in range(cap):
-                _warn_cgroup_fallback_once("CPUPlugin", f"filler_{i}")
-            # `first_cid` should have been evicted and now warn again.
-            _warn_cgroup_fallback_once("CPUPlugin", first_cid)
-
-        warn_records = [r for r in caplog.records if r.levelname == "WARNING"]
-        # 1 (first) + cap (fillers) + 1 (re-warn of first) = cap + 2
-        assert len(warn_records) == cap + 2
-
 
 class TestMemoryPluginContainerPidValidation(BaseDockerIntrinsicTest):
     """Tests for container PID validation before reading /proc/[pid]/net/dev."""