test: cover gateway session model backfill

Add regression coverage for backfilling NULL gateway session models in SQLite, preserving existing models, and forwarding the resolved agent model through SessionStore updates.

tests/gateway/test_session.py

@@ -577,3 +577,28 @@ class TestLastPromptTokens:
 
         store.update_session("k1", last_prompt_tokens=0)
         assert entry.last_prompt_tokens == 0
+
+    def test_update_session_passes_model_to_db(self, tmp_path):
+        """Gateway session updates should forward the resolved model to SQLite."""
+        config = GatewayConfig()
+        with patch("gateway.session.SessionStore._ensure_loaded"):
+            store = SessionStore(sessions_dir=tmp_path, config=config)
+        store._loaded = True
+        store._save = MagicMock()
+        store._db = MagicMock()
+
+        from gateway.session import SessionEntry
+        from datetime import datetime
+        entry = SessionEntry(
+            session_key="k1",
+            session_id="s1",
+            created_at=datetime.now(),
+            updated_at=datetime.now(),
+        )
+        store._entries = {"k1": entry}
+
+        store.update_session("k1", model="openai/gpt-5.4")
+
+        store._db.update_token_counts.assert_called_once_with(
+            "s1", 0, 0, model="openai/gpt-5.4"
+        )