fix(providers): report Responses cached tokens

nanobot/providers/openai_responses/parsing.py

@@ -203,11 +203,16 @@ def _usage_from_response_obj(response: object) -> dict[str, int]:
         usage.get("output_tokens") or usage.get("completion_tokens") or 0
     )
     total_tokens = int(usage.get("total_tokens") or prompt_tokens + completion_tokens)
-    return {
+    result = {
         "prompt_tokens": prompt_tokens,
         "completion_tokens": completion_tokens,
         "total_tokens": total_tokens,
     }
+    input_details = _response_object(usage.get("input_tokens_details"))
+    cached_tokens = int(input_details.get("cached_tokens") or 0) if input_details else 0
+    if cached_tokens > 0:
+        result["cached_tokens"] = cached_tokens
+    return result
 
 
 def _parse_tool_call_arguments(args_raw: Any, name: str | None) -> Any:
@@ -789,6 +794,13 @@ async def consume_sdk_stream(
                         "completion_tokens": int(getattr(usage_obj, "output_tokens", 0) or 0),
                         "total_tokens": int(getattr(usage_obj, "total_tokens", 0) or 0),
                     }
+                    usage_data = _response_object(usage_obj) or {}
+                    input_details = _response_object(usage_data.get("input_tokens_details"))
+                    cached_tokens = (
+                        int(input_details.get("cached_tokens") or 0) if input_details else 0
+                    )
+                    if cached_tokens > 0:
+                        usage["cached_tokens"] = cached_tokens
                 if not reasoning_content:
                     reasoning_content = _extract_reasoning_summary_from_output(
                         getattr(resp, "output", None)