fix: coerce None token fields to 0 in OpenAIHelper.get_ai_usage_from_response; update LangChain README example

Author: jsonbailey

packages/ai-providers/server-ai-openai/src/ldai_openai/openai_helper.py

@@ -38,9 +38,9 @@ def get_ai_usage_from_response(response: Any) -> Optional[TokenUsage]:
         if hasattr(response, 'usage') and response.usage is not None:
             u = response.usage
             usage = TokenUsage(
-                total=getattr(u, 'total_tokens', 0),
-                input=getattr(u, 'prompt_tokens', 0),
-                output=getattr(u, 'completion_tokens', 0),
+                total=getattr(u, 'total_tokens', None) or 0,
+                input=getattr(u, 'prompt_tokens', None) or 0,
+                output=getattr(u, 'completion_tokens', None) or 0,
             )
         if usage is not None and usage.total == 0 and usage.input == 0 and usage.output == 0:
             return None