test(agent): Cover empty allocation and clarify k8s/dummy NUMA skip

Post-review follow-ups on PR #11222:
- Add an `empty_allocation` scenario to `_NUMA_SCENARIOS` so the `cores=[]`
  path through `_resolve_node_local_mem` / `generate_docker_args` is locked
  in (must not emit `CpusetMems`).
- Document in `kubernetes/intrinsic.py::CPUPlugin.generate_docker_args` why
  the NUMA pinning from the Docker backend is not mirrored (k8s drives
  node-local memory via the kubelet Topology Manager).
- Document in `dummy/intrinsic.py::CPUPlugin.generate_docker_args` why
  dummy intentionally skips the NUMA pinning (never runs real containers).

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

Author: rapsealk

src/ai/backend/agent/dummy/intrinsic.py

@@ -137,6 +137,10 @@ async def generate_docker_args(
         docker: aiodocker.docker.Docker,
         device_alloc: DeviceAllocation,
     ) -> Mapping[str, Any]:
+        # The Docker backend pins ``CpusetMems`` to the allocation's NUMA node
+        # when the allocation is node-local. The dummy backend intentionally
+        # skips that because it never actually runs containers; the output is
+        # only inspected by tests that exercise the plumbing, not NUMA policy.
         cores = [*map(int, device_alloc[SlotName("cpu")].keys())]
         sorted_core_ids = [*map(str, sorted(cores))]
         return {

src/ai/backend/agent/kubernetes/intrinsic.py

@@ -191,7 +191,10 @@ async def generate_docker_args(
         device_alloc: Mapping[SlotName, Mapping[DeviceId, Decimal]],
     ) -> Mapping[str, Any]:
         # This function might be needed later to apply fine-grained tuning for
-        # K8s resource allocation
+        # K8s resource allocation. NUMA memory pinning is intentionally not
+        # mirrored from the Docker backend here: on Kubernetes, node-local
+        # memory placement is driven by the kubelet Topology Manager
+        # (via pod-level resource hints), not via per-container HostConfig.
         return {}
 
     async def restore_from_container(

tests/unit/agent/test_docker_intrinsic.py

@@ -683,6 +683,15 @@ class _NumaScenario:
         cores=[0, 1],
         expected_cpuset_mems=None,
     ),
+    # Empty allocation → no cores to pin; must not produce CpusetMems="" or
+    # a spurious node id. Locks in the defensive behavior so a future
+    # refactor of _resolve_node_local_mem cannot regress it.
+    "empty_allocation": _NumaScenario(
+        num_nodes=2,
+        core_to_node={0: 0, 1: 0},
+        cores=[],
+        expected_cpuset_mems=None,
+    ),
 }