[ui] Fix internal static asset test for GraphiQL (#23448)

## Summary & Motivation

Fix internal test related to the GraphiQL change. The relevant
configuration is in OSS, but the test runs internally.

## How I Tested These Changes

Point internal branch at this branch, verify that the internal build
passes.

python_modules/dagster-webserver/dagster_webserver/webserver.py

@@ -292,6 +292,10 @@ def _static_file(path, file_path):
             # These are tested by an internal test without building the app.
             return [
                 Route("/favicon.png", lambda _: FileResponse(path="/favicon")),
+                Route(
+                    "/vendor/graphiql/graphiql.min.css",
+                    lambda _: FileResponse(path="/vendor/graphiql/graphiql.min.css"),
+                ),
             ]
 
         return routes