fix(tests): satisfy pyright strict on the overlap-test helper

The TestNetpoolOverlap `_routes` helper had an untyped fake_run + a bare
`dict`/`object` return, tripping 9 strict-mode errors (missing type args,
missing param annotations, `object` not usable as a context manager).
Type it properly: entries: list[dict[str, str]], fake_run params, and a
`AbstractContextManager[MagicMock]` return. `pyright` is clean again.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

tests/unit/test_firecracker_backend.py

@@ -8,11 +8,14 @@ the VM boot-arg assembly.
 from __future__ import annotations
 
 import base64
+import json
 import os
+import subprocess
 import unittest
+from contextlib import AbstractContextManager
 from pathlib import Path
 from typing import Any, cast
-from unittest.mock import patch
+from unittest.mock import MagicMock, patch
 
 from bot_bottle.backend.firecracker import firecracker_vm, netpool
 from bot_bottle.backend.firecracker.bottle import FirecrackerBottle
@@ -137,11 +140,12 @@ class TestNetpoolOverlap(unittest.TestCase):
     existing host route (Tailscale CGNAT peer, docker/libvirt bridge,
     LAN) and ignores our own bbfc TAPs + the default route."""
 
-    def _routes(self, entries: list[dict]) -> object:
+    def _routes(
-        import json
+        self, entries: list[dict[str, str]]
-        import subprocess
+    ) -> AbstractContextManager[MagicMock]:
-
+        def fake_run(
-        def fake_run(argv, **kwargs):
+            argv: list[str], **kwargs: object
+        ) -> subprocess.CompletedProcess[str]:
             return subprocess.CompletedProcess(
                 argv, 0, stdout=json.dumps(entries), stderr="",
             )