fix(rag): send explicit encoding_format to avoid llama.cpp null error

When encoding_format is unset, LiteLLM forwards it to the backend as JSON
null, and llama.cpp's embeddings endpoint rejects it with a 500
("type must be string, but is null"). Pin encoding_format="float" so the
gateway always relays a string.

packages/rag/default.nix

@@ -34,7 +34,9 @@ writers.writePython3Bin "rag"
 
 
     def embed(texts):
-        resp = client.embeddings.create(model=EMBED_MODEL, input=texts)
+        # encoding_format is explicit: llama.cpp rejects a null value, and
+        # LiteLLM forwards an unset one as JSON null.
+        resp = client.embeddings.create(model=EMBED_MODEL, input=texts, encoding_format="float")
         return [d.embedding for d in resp.data]