tests: serialize the router tests that download the same model

Parallel workers share one cache, so the two tests fetch the same blob
into the same in-progress file and race to rename it. They now take a
file lock around the download, like the session fixture does for the
preset models.

tools/server/tests/unit/test_router.py

@@ -1,5 +1,6 @@
 import threading
 import pytest
+from filelock import FileLock
 from utils import *
 
 server: ServerProcess
@@ -504,6 +505,18 @@ MODEL_DOWNLOAD_ID = "ggml-org/test-model-router-download:F16"
 MODEL_DOWNLOAD_TIMEOUT = 30
 
 
+@pytest.fixture
+def download_model_lock(tmp_path_factory):
+    """Serialize the tests that download MODEL_DOWNLOAD_ID.
+
+    Parallel workers share one cache, so they would fetch the same blob into
+    the same in-progress file and race to rename it.
+    """
+    root_tmp_dir = tmp_path_factory.getbasetemp().parent
+    with FileLock(str(root_tmp_dir / "model_download.lock")):
+        yield
+
+
 def _listen_sse(
     server: ServerProcess, collected: list, stop: threading.Event, ready: threading.Event | None = None
 ):
@@ -539,7 +552,7 @@ def _wait_for_sse_event(collected: list, event_type: str, model: str, timeout: i
     return False
 
 
-def test_router_download_model():
+def test_router_download_model(download_model_lock):
     """Case 1: download a model, verify SSE events and GET /models."""
     global server
     server.start()
@@ -582,7 +595,7 @@ def test_router_download_model():
     assert MODEL_DOWNLOAD_ID in ids, f"{MODEL_DOWNLOAD_ID} not found in /models after download"
 
 
-def test_router_delete_model():
+def test_router_delete_model(download_model_lock):
     """Case 2: delete the downloaded model, verify it disappears from GET /models."""
     global server
     server.start()