fix: resolve the code-review findings on the kernel datapath

Compute:
- Track the anchor that owns uBridge's single AF_PACKET capture slot: a
  second kernel port's capture is refused with a clear error (its flag
  rolled back), a failed start no longer wedges the port, and only the
  owning port's stop issues the process-wide `capture stop_kernel`
  (docker/qemu/iou/dynamips/switch, via the mixin's slot helpers).
- Unwind failed starts instead of leaking uBridge and the anchor TAPs:
  QEMU's stop body runs under try/finally, QEMU and IOU start the datapath
  inside the launch try, Dynamips undoes a half-attached port (the retry
  used to be short-circuited into a silent black hole), and Docker deletes
  the veth pair when the namespace move fails.
- Restore the relay filter guard on IOU (kernel-only filters raise
  NodeError, an uncompilable BPF line degrades to a warning), validate the
  eBPF/quota/window/bpf filter shapes before touching the bridge (short or
  non-numeric values surfaced as 500s after partial state), and refuse a
  second NIO on an occupied Docker/QEMU port (the guard IOU and Dynamips
  already had).
- Probe fixes: `_probe_bridge_tap` survives a failing uBridge start, and
  the probe caches key on ctime too, so a chmod/setcap remedy re-probes.

Controller:
- Parse GNS3_UNIX_SOCKET_NIO like the compute does (line-anchored, truthy
  value) and mirror the compute's class selection in kernel_anchor_type.
- Read partial capability reports defensively; make BridgeNIO.bridge
  required-but-nullable so an omitted key is a 422 rather than a KeyError.
- Strip the packet_loss correlation slot even when it is zero: the relay
  filter takes exactly one argument and rejected `packet_loss 10 0`.

Ethernet switch: skip ports whose absorbed anchor does not exist yet (the
join is deferred by design) and roll the ports_mapping back when the VLAN
reconcile fails, so a retry still diffs against the old mapping.

Agent: reject device config entries without a device_name instead of
silently dropping them (a malformed batch used to execute partially while
reporting success).

Also split the netem builder to clear C901 and drop the two per-file ruff
ignores the PR had added, keeping the remove-only policy.

gns3server/utils/kernel_anchor.py

@@ -59,12 +59,18 @@ def kernel_anchor_type(node_type, environment=None):
     its own key unchanged.
     """
 
-    # The marker check mirrors utils.application_id.is_iol_runner_environment
-    # inline: this module sits below every node module (compute and
-    # controller alike import it during their own import), so it must not
-    # depend on anything that pulls the controller package in.
-    if node_type == "docker" and "GNS3_IOL_RUNNER=" in (environment or ""):
-        return "iol_docker"
+    # The marker check mirrors the compute's class selection
+    # (compute/docker/__init__.py, _select_node_class) inline: only a
+    # stripped, comma-trimmed line *beginning* with the marker selects
+    # IOLDockerVM, so e.g. NOTE=GNS3_IOL_RUNNER=1 must not be named as an
+    # IOL TAP anchor here. The check stays dependency-free on purpose: this
+    # module sits below every node module (compute and controller alike
+    # import it during their own import), so it must not import them back.
+    if node_type == "docker":
+        for line in (environment or "").splitlines():
+            line = line.strip().rstrip(",")
+            if line.startswith("GNS3_IOL_RUNNER="):
+                return "iol_docker"
     return node_type