test: add region to the fake browser fixture

The API spec gains region as a required field on browser responses
(kernel/kernel#3270); strict response validation rejects the fixture
without it. Safe against the current spec — models allow extra keys.

Author: tnsardesai

tests/test_browser_routing.py

@@ -32,6 +32,7 @@ def _fake_browser() -> dict[str, object]:
         "headless": True,
         "stealth": False,
         "timeout_seconds": 60,
+        "region": "us-east",
     }