fix: backfill model on gateway sessions after agent runs

Gateway sessions end up with model=NULL because the session row is
created before AIAgent is constructed.  After the agent responds,
update_session() writes token counts but never fills in the model.

Thread agent.model through _run_agent()'s return dict into
update_session() → update_token_counts().  The SQL uses
COALESCE(model, ?) so it only fills NULL rows — never overwrites
a model already set at creation time (e.g. CLI sessions).

If the agent falls back to a different provider, agent.model is
updated in-place by _try_activate_fallback(), so the recorded value
reflects whichever model actually produced the response.

Fixes #987

gateway/run.py

@@ -1578,10 +1578,11 @@ class GatewayRunner:
                         skip_db=agent_persisted,
                     )
             
-            # Update session with actual prompt token count from the agent
+            # Update session with actual prompt token count and model from the agent
             self.session_store.update_session(
                 session_entry.session_key,
                 last_prompt_tokens=agent_result.get("last_prompt_tokens", 0),
+                model=agent_result.get("model"),
             )
             
             return response
@@ -3586,6 +3587,7 @@ class GatewayRunner:
                     "tools": tools_holder[0] or [],
                     "history_offset": len(agent_history),
                     "last_prompt_tokens": _last_prompt_toks,
+                    "model": agent_holder[0].model if agent_holder[0] else None,
                 }
             
             # Scan tool results for MEDIA:<path> tags that need to be delivered
@@ -3648,6 +3650,7 @@ class GatewayRunner:
                 "tools": tools_holder[0] or [],
                 "history_offset": len(agent_history),
                 "last_prompt_tokens": _last_prompt_toks,
+                "model": agent_holder[0].model if agent_holder[0] else None,
                 "session_id": effective_session_id,
             }