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

.github/workflows/server-sanitize.yml

@@ -103,7 +103,7 @@ jobs:
           source .venv/bin/activate
           cd tools/server/tests
           export ${{ matrix.extra_args }}
-          ./tests.sh
+          PYTEST_WORKERS=1 ./tests.sh
 
       - name: Slow tests
         id: server_integration_tests_slow
@@ -112,4 +112,4 @@ jobs:
           source .venv/bin/activate
           cd tools/server/tests
           export ${{ matrix.extra_args }}
-          SLOW_TESTS=1 ./tests.sh
+          PYTEST_WORKERS=1 SLOW_TESTS=1 ./tests.sh