server : use pytest-xdist for server tests (#28298)

* server : use pytest-xdist for server tests

This commit adds pytest-xdist to the server tests. This is pytest
plugin that distributes test execution across multiple CPU cores.

Assisted-by: pi:llama.cpp/qwen3.8-27B

Refs: https://github.com/ggml-org/llama.cpp/pull/26734#issuecomment-5220707042

* remove server_base_port and BASE_PORT

* use worksteal and pytest builting tmp_path

tools/server/tests/conftest.py

@@ -1,7 +1,17 @@
+import os
 import pytest
+from filelock import FileLock
 from utils import *
 
 
+@pytest.fixture(scope="session", autouse=True)
+def configure_worker_port(request):
+    worker_id = getattr(request.config, "workerinput", {}).get("workerid", "master")
+    if worker_id != "master":
+        worker_num = int(worker_id[2:])
+        os.environ["PORT"] = str(8080 + worker_num * 10)
+
+
 # ref: https://stackoverflow.com/questions/22627659/run-code-before-and-after-each-test-in-py-test
 @pytest.fixture(autouse=True)
 def stop_server_after_each_test():
@@ -16,6 +26,10 @@ def stop_server_after_each_test():
 
 
 @pytest.fixture(scope="session", autouse=True)
-def load_server_presets():
+def load_server_presets(configure_worker_port, tmp_path_factory):
     # this will be run once per test session, before any tests
-    ServerPreset.load_all()
+
+    # serialize model downloads across parallel workers.
+    root_tmp_dir = tmp_path_factory.getbasetemp().parent
+    with FileLock(str(root_tmp_dir / "load_all.lock")):
+        ServerPreset.load_all()