nixos/open-webui: fix opensearch

When [adding Open WebUI as search engine in Firefox](https://docs.openwebui.com/tutorials/integrations/browser-search-engine/#for-firefox) it always links to `http://localhost:3000` instead of the configured port

It is because [the response use `WEBUI_URL` environment variable](1d225dd804/backend/open_webui/main.py (L2370)) which [is set by default to `1d225dd804/backend/open_webui/env.py (L106))

nixos/modules/services/misc/open-webui.nix

@@ -93,6 +93,7 @@ in
         DATA_DIR = ".";
         HF_HOME = ".";
         SENTENCE_TRANSFORMERS_HOME = ".";
+        WEBUI_URL = "http://localhost:${toString cfg.port}";
       } // cfg.environment;
 
       serviceConfig = {

nixos/tests/open-webui.nix

@@ -31,6 +31,7 @@ in
 
   testScript = ''
     import json
+    import xml.etree.ElementTree as xml
 
     machine.start()
 
@@ -45,5 +46,18 @@ in
 
     # Check that the name was overridden via the environmentFile option.
     assert webui_config["name"] == "${webuiName} (Open WebUI)"
+
+    webui_opensearch_xml = machine.succeed("curl http://127.0.0.1:${mainPort}/opensearch.xml")
+    webui_opensearch = xml.fromstring(webui_opensearch_xml)
+
+    webui_opensearch_url = webui_opensearch.find(
+        ".//{http://a9.com/-/spec/opensearch/1.1/}Url"
+    )
+    assert (
+        webui_opensearch_url is not None
+    ), f"no url tag found in {webui_opensearch_xml}"
+    assert (
+        webui_opensearch_url.get("template") == "http://localhost:8080/?q={searchTerms}"
+    ), "opensearch url doesn't match the configured port"
   '';
 }