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/unit/test_mcp_servers.py

@@ -37,7 +37,6 @@ def _start_server_with_mcp(mcp_json: str, **kwargs) -> ServerProcess:
     srv = ServerPreset.router()
     srv.server_tools = "all"
     srv.no_ui = True
-    srv.server_port = 8085  # avoid conflict with load_all() which uses 8080
     srv.mcp_servers_json = mcp_json
     for k, v in kwargs.items():
         setattr(srv, k, v)
@@ -183,7 +182,6 @@ def test_mcp_tools_not_listed_when_not_configured():
     server = ServerPreset.router()
     server.server_tools = "all"
     server.no_ui = True
-    server.server_port = 8085
     server.start()
 
     try:
@@ -250,7 +248,6 @@ def test_mcp_tools_via_json_config_file():
         server = ServerPreset.router()
         server.server_tools = "all"
         server.no_ui = True
-        server.server_port = 8085
         server.mcp_servers_config = config_path
         server.start()
 
@@ -468,7 +465,6 @@ def test_mcp_config_file_errors():
     server = ServerPreset.router()
     server.server_tools = "all"
     server.no_ui = True
-    server.server_port = 8085
     server.mcp_servers_json = "not valid json"
     try:
         server.start()
@@ -480,7 +476,6 @@ def test_mcp_config_file_errors():
     server = ServerPreset.router()
     server.server_tools = "all"
     server.no_ui = True
-    server.server_port = 8085
     server.mcp_servers_config = "/nonexistent/path.json"
     try:
         server.start()