fix(sgx): pull litellm from unstable for embeddings fix

LiteLLM 1.75.5 in nixpkgs 25.11 crashes on /v1/embeddings with
"RuntimeError: Stream consumed" (the proxy reads the request body twice).
Source litellm from nixos-unstable (1.83.14) where this is resolved, so
the gateway can proxy embeddings to halo's bge-m3 for any client.

overlays/unstable/default.nix

@@ -7,6 +7,7 @@ final: prev: {
     mistral-vibe
     claude-code
     qwen-code
+    litellm # 25.11's 1.75.5 has a /v1/embeddings "Stream consumed" bug
     # llama-cpp-rocm
     # open-webui
     # vscode