feat(agent): add redis short-term user context cache and align tests

backend/tests/unit/database/test_sessions_state_snapshot_contract.py

@@ -27,3 +27,13 @@ def test_message_has_token_cost_and_metadata_contract() -> None:
     versions_dir = Path(__file__).resolve().parents[3] / "alembic" / "versions"
     migration_file = versions_dir / "20260305_agent_runtime_closed_loop_contract.py"
     assert migration_file.exists()
+
+
+def test_message_currency_removal_migration_contract() -> None:
+    versions_dir = Path(__file__).resolve().parents[3] / "alembic" / "versions"
+    migration_file = versions_dir / "20260306_0002_drop_message_currency.py"
+    assert migration_file.exists()
+
+    content = migration_file.read_text(encoding="utf-8")
+    assert "drop_column(\"messages\", \"currency\")" in content
+    assert "Irreversible migration" in content