open-webui: test environmentFile option

Modify the tests for open-webui such that the name of the service
is set via the 'environmentFile' option, then check that the
service's name differs from the default.

nixos/tests/open-webui.nix

@@ -1,6 +1,7 @@
-{ lib, ... }:
+{ config, lib, ... }:
 let
   mainPort = "8080";
+  webuiName = "NixOS Test";
 in
 {
   name = "open-webui";
@@ -18,16 +19,31 @@ in
             # Requires network connection
             RAG_EMBEDDING_MODEL = "";
           };
+
+          # Test that environment variables can be
+          # overridden through a file.
+          environmentFile = config.node.pkgs.writeText "test.env" ''
+            WEBUI_NAME="${webuiName}"
+          '';
         };
       };
   };
 
   testScript = ''
+    import json
+
     machine.start()
 
     machine.wait_for_unit("open-webui.service")
     machine.wait_for_open_port(${mainPort})
 
     machine.succeed("curl http://127.0.0.1:${mainPort}")
+
+    # Load the Web UI config JSON and parse it.
+    webui_config_json = machine.succeed("curl http://127.0.0.1:${mainPort}/api/config")
+    webui_config = json.loads(webui_config_json)
+
+    # Check that the name was overridden via the environmentFile option.
+    assert webui_config["name"] == "${webuiName} (Open WebUI)"
   '';
 }