fix: repair imports lost in the 3.1 rebase and meet the new lint baseline

The rebase onto upstream 3.1 (mypy + ruff I001/F401 waves) left the tree
short of the newly enforced baselines in three ways:

* contextlib / UbridgeError imports dropped as false duplicates during
  conflict resolution left NameErrors in iou_vm and qemu_vm (F821);
  the Dynamips router grabbed the compute-level NIOTAP instead of the
  hypervisor one, breaking TAP anchoring (test failure).
* unused imports (KERNEL_UNSUPPORTED_FILTERS, UbridgeError, List) and
  unsorted blocks left behind where the stack's later commits moved the
  last usage elsewhere — ruff I001/F401/RUF022 now enforce these.
* mypy, now baseline-free: cache dict annotations in tc_probe, the
  locators candidate list, the resolved-params None branch in the MCP
  runner, and validate_bpf_syntax keeps the upstream Dict[str, Any]
  return (the strict Optional[str] was wrong for the "valid" bool entry).

The e2e raw-frame probe resolves its compiler via shutil.which (S607)
and zips with strict= (B905); the remaining subprocess call is S603 by
nature and gets a per-file ignore like other test files.

ruff check/format clean, mypy at upstream parity (no new errors),
full unit suite 2167 passed.

tests/controller/test_compute.py

@@ -381,7 +381,18 @@ async def test_json(compute):
         "disk_usage_percent": 0,
         "connected": True,
         "last_error": None,
-        "capabilities": {"version": "", "platform": "", "cpus": 0, "memory": 0, "disk_size": 0, "node_types": [], "ubridge_tc": None, "ubridge_tap": None, "ubridge_iol_tap": None, "ubridge_bridge_tap": None},
+        "capabilities": {
+            "version": "",
+            "platform": "",
+            "cpus": 0,
+            "memory": 0,
+            "disk_size": 0,
+            "node_types": [],
+            "ubridge_tc": None,
+            "ubridge_tap": None,
+            "ubridge_iol_tap": None,
+            "ubridge_bridge_tap": None,
+        },
     }
     assert compute.asdict(topology_dump=True) == {
         "compute_id": "my_compute_id",

tests/controller/test_kernel_datapath_link.py

@@ -20,14 +20,15 @@ Controller-side kernel-datapath link selection: eligibility of Docker-to-
 Docker links, NIO emission and the filters/markers guards.
 """
 
-import pytest
 from unittest.mock import MagicMock, patch
 
+import pytest
+
 from gns3server.config import Config
-from gns3server.controller.udp_link import UDPLink
-from gns3server.controller.ports.ethernet_port import EthernetPort
-from gns3server.controller.node import Node
 from gns3server.controller.controller_error import ControllerError
+from gns3server.controller.node import Node
+from gns3server.controller.ports.ethernet_port import EthernetPort
+from gns3server.controller.udp_link import UDPLink
 
 
 def _node(project, compute, name, node_type="docker", status="stopped", environment=None):