Include API prefix in thing_descriptions endpoint.

Individual thing descriptions were being generated correctly, but the combined endpoint passed the wrong path to the `thing_description` methods.

This should fix the form URLs included in that endpoint, which is very important for the web app.

Author: rwb27

src/labthings_fastapi/server/__init__.py

@@ -363,7 +363,9 @@ def thing_descriptions(request: Request) -> Mapping[str, ThingDescription]:
                 dictionaries.
             """
             return {
-                name: thing.thing_description(name + "/", base=str(request.base_url))
+                name: thing.thing_description(
+                    path=f"{self._api_prefix}/{name}/", base=str(request.base_url)
+                )
                 for name, thing in thing_server.things.items()
             }