feat: show token usage per assistant response (#2270)

* feat: show token usage per assistant response

* fix: align client models response with token usage

* fix: address token usage review feedback

* docs: clarify token usage config example

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/packages/harness/deerflow/client.py

@@ -722,6 +722,10 @@ class DeerFlowClient:
             Dict with "models" key containing list of model info dicts,
             matching the Gateway API ``ModelsListResponse`` schema.
         """
+        token_usage_enabled = getattr(getattr(self._app_config, "token_usage", None), "enabled", False)
+        if not isinstance(token_usage_enabled, bool):
+            token_usage_enabled = False
+
         return {
             "models": [
                 {
@@ -733,7 +737,8 @@ class DeerFlowClient:
                     "supports_reasoning_effort": getattr(model, "supports_reasoning_effort", False),
                 }
                 for model in self._app_config.models
-            ]
+            ],
+            "token_usage": {"enabled": token_usage_enabled},
         }
 
     def list_skills(self, enabled_only: bool = False) -> dict: