feat(agent): complete closed-loop runtime and pricing fallback

backend/tests/unit/database/test_sessions_state_snapshot_contract.py

@@ -0,0 +1,29 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+
+def test_session_has_state_snapshot_and_status_contract() -> None:
+    model_path = (
+        Path(__file__).resolve().parents[3] / "src" / "models" / "agent_chat_session.py"
+    )
+    content = model_path.read_text(encoding="utf-8")
+
+    assert "state_snapshot" in content
+    assert "AgentChatSessionStatus" in content
+
+
+def test_message_has_token_cost_and_metadata_contract() -> None:
+    model_path = (
+        Path(__file__).resolve().parents[3] / "src" / "models" / "agent_chat_message.py"
+    )
+    content = model_path.read_text(encoding="utf-8")
+
+    assert "input_tokens" in content
+    assert "output_tokens" in content
+    assert "cost" in content
+    assert '"metadata"' in content
+
+    versions_dir = Path(__file__).resolve().parents[3] / "alembic" / "versions"
+    migration_file = versions_dir / "20260305_agent_runtime_closed_loop_contract.py"
+    assert migration_file.exists()