feat(agent-chat): complete core workflow and strengthen auth rate limiting

backend/tests/unit/database/test_agent_chat_migration_contract.py

@@ -0,0 +1,17 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+
+def test_agent_chat_migration_exists_and_creates_expected_tables() -> None:
+    versions_dir = Path(__file__).resolve().parents[3] / "alembic" / "versions"
+    migration = versions_dir / "20260226_create_agent_chat_core_tables.py"
+
+    assert migration.exists()
+
+    content = migration.read_text(encoding="utf-8")
+    assert 'create_table(\n        "llm_factory"' in content
+    assert 'create_table(\n        "llms"' in content
+    assert 'create_table(\n        "sessions"' in content
+    assert 'create_table(\n        "messages"' in content
+    assert "tool_calls" not in content