fix(providers): handle empty choices in custom provider response

nanobot/providers/custom_provider.py

@@ -54,6 +54,11 @@ class CustomProvider(LLMProvider):
             return LLMResponse(content=f"Error: {e}", finish_reason="error")
 
     def _parse(self, response: Any) -> LLMResponse:
+        if not response.choices:
+            return LLMResponse(
+                content="Error: API returned empty choices. This may indicate a temporary service issue or an invalid model response.",
+                finish_reason="error"
+            )
         choice = response.choices[0]
         msg = choice.message
         tool_calls = [