fix: graceful fallback when langfuse is not installed

- Use import importlib.util (not bare importlib) for find_spec
- Warn and fall back to standard openai instead of crashing with
  ImportError when LANGFUSE_SECRET_KEY is set but langfuse is missing

Made-with: Cursor

nanobot/providers/openai_compat_provider.py

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 import asyncio
 import hashlib
-import importlib
+import importlib.util
 import os
 import secrets
 import string
@@ -14,13 +14,15 @@ from typing import TYPE_CHECKING, Any
 
 import json_repair
 
-if os.environ.get("LANGFUSE_SECRET_KEY"):
+if os.environ.get("LANGFUSE_SECRET_KEY") and importlib.util.find_spec("langfuse"):
-    LANGFUSE_AVAILABLE  = importlib.util.find_spec("langfuse") is not None
-    if not LANGFUSE_AVAILABLE:
-        raise ImportError("Langfuse is not available; please install it with `pip install langfuse`")
-
     from langfuse.openai import AsyncOpenAI
 else:
+    if os.environ.get("LANGFUSE_SECRET_KEY"):
+        import logging
+        logging.getLogger(__name__).warning(
+            "LANGFUSE_SECRET_KEY is set but langfuse is not installed; "
+            "install with `pip install langfuse` to enable tracing"
+        )
     from openai import AsyncOpenAI
 
 from nanobot.providers.base import LLMProvider, LLMResponse, ToolCallRequest