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/compute/iou/test_iou_kernel_datapath.py

@@ -22,19 +22,16 @@ it, kernel-link enslavement, the relay fallback for a uBridge without the
 command, and the reverse-of-QEMU stop order (uBridge holds the anchor fds).
 """
 
+from unittest.mock import MagicMock, patch
+
 import pytest
 import pytest_asyncio
 
-from unittest.mock import MagicMock, patch
-
-from tests.utils import AsyncioMagicMock
-
 from gns3server.compute.iou import IOU
-from gns3server.compute.iou.iou_vm import IOUVM
 from gns3server.compute.iou.iou_error import IOUError
-from gns3server.compute.nios.nio_bridge import NIOBridge
+from gns3server.compute.iou.iou_vm import IOUVM
 from gns3server.compute.ubridge.ubridge_error import UbridgeError
-
+from tests.utils import AsyncioMagicMock
 
 NODE_ID = "00010203-0405-0607-0809-0a0b0c0d0e0f"
 BRIDGE = "gns3a1b2c3d4e5f"