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.

gns3server/compute/iou/iou_vm.py

@@ -22,6 +22,7 @@ order to run an IOU VM.
 import asyncio
 import binascii
 import configparser
+import contextlib
 import functools
 import glob
 import hashlib
@@ -951,9 +952,7 @@ class IOUVM(KernelDatapathMixin, BaseNode):
             raise self._kernel_error(
                 "Bay {bay}/{unit} of IOU '{name}' has no TAP anchor to carry a kernel link "
                 "(serial ports stay on the relay datapath, and a node whose uBridge lacks "
-                "iol_bridge add_nio_tap runs relay-only)".format(
-                    bay=adapter_number, unit=port_number, name=self._name
-                )
+                "iol_bridge add_nio_tap runs relay-only)".format(bay=adapter_number, unit=port_number, name=self._name)
             )
         await self._ubridge_send(
             'iol_bridge add_nio_tap {name} {iol_id} {bay} {unit} "{tap}"'.format(