feat(session): include user_id and total_cost_usd in trajectory dump

Mirror of huggingface/ml-intern#136 applied surgically to the Space's
own main branch (the github/main merge has unrelated drift that would
pull in too much).

Add user_id (HF username from OAuth) to the Session object and
propagate it from the SessionManager. Surface it in the trajectory
JSON written by save_trajectory_local() and uploaded to the session
dataset, alongside a total_cost_usd aggregate summed from existing
llm_call events.

agent/core/session.py

@@ -79,8 +79,10 @@ class Session:
         hf_token: str | None = None,
         local_mode: bool = False,
         stream: bool = True,
+        user_id: str | None = None,
     ):
         self.hf_token: Optional[str] = hf_token
+        self.user_id: Optional[str] = user_id
         self.tool_router = tool_router
         self.stream = stream
         tool_specs = tool_router.get_tool_specs_for_llm() if tool_router else []
@@ -199,11 +201,21 @@ class Session:
                 tools = self.tool_router.get_tool_specs_for_llm() or []
             except Exception:
                 tools = []
+        # Sum per-call cost from llm_call events so analyzers don't have to
+        # walk the events array themselves. Each `llm_call` event already
+        # carries cost_usd from `agent.core.telemetry.record_llm_call`.
+        total_cost_usd = sum(
+            float((e.get("data") or {}).get("cost_usd") or 0.0)
+            for e in self.logged_events
+            if e.get("event_type") == "llm_call"
+        )
         return {
             "session_id": self.session_id,
+            "user_id": self.user_id,
             "session_start_time": self.session_start_time,
             "session_end_time": datetime.now().isoformat(),
             "model_name": self.config.model_name,
+            "total_cost_usd": total_cost_usd,
             "messages": [msg.model_dump() for msg in self.context_manager.items],
             "events": self.logged_events,
             "tools": tools,

backend/session_manager.py

@@ -192,7 +192,7 @@ class SessionManager:
                 session_config.model_name = model
             session = Session(
                 event_queue, config=session_config, tool_router=tool_router,
-                hf_token=hf_token,
+                hf_token=hf_token, user_id=user_id,
             )
             t1 = _time.monotonic()
             logger.info(f"Session initialized in {t1 - t0:.2f}s")