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/kernel_datapath.py

@@ -41,17 +41,16 @@ A node type using this mixin provides:
   failures surface as ``DockerError`` / ``QemuError`` rather than ``NodeError``.
 """
 
-import os
 import contextlib
+import logging
+import os
 
-from gns3server.utils.tc_capabilities import FILTER_EBPF_MODES, parse_tc_capabilities, usable_ebpf_modes
-from gns3server.compute.ubridge.ubridge_error import UbridgeError
 from gns3server.compute.error import NodeError
+from gns3server.compute.ubridge.ubridge_error import UbridgeError
+from gns3server.utils.tc_capabilities import FILTER_EBPF_MODES, parse_tc_capabilities, usable_ebpf_modes
 
 from .nios.nio_bridge import NIOBridge
 
-import logging
-
 log = logging.getLogger(__name__)