fix(session): make SQLite transcript rewrites transactional

tests/gateway/test_session.py

@@ -1233,3 +1233,34 @@ class TestRewriteTranscriptPreservesReasoning:
         assert after[0].get("reasoning_content") == "provider scratchpad"
         assert after[0].get("reasoning_details") == [{"type": "summary", "text": "step by step"}]
         assert after[0].get("codex_reasoning_items") == [{"id": "r1", "type": "reasoning"}]
+
+    def test_db_rewrite_is_atomic_on_insert_failure(self, tmp_path):
+        from hermes_state import SessionDB
+
+        db = SessionDB(db_path=tmp_path / "test.db")
+        session_id = "atomic-rewrite-test"
+        db.create_session(session_id=session_id, source="cli")
+        db.append_message(session_id=session_id, role="user", content="before user")
+        db.append_message(session_id=session_id, role="assistant", content="before assistant")
+
+        config = GatewayConfig()
+        with patch("gateway.session.SessionStore._ensure_loaded"):
+            store = SessionStore(sessions_dir=tmp_path, config=config)
+        store._db = db
+        store._loaded = True
+
+        replacement = [
+            {"role": "user", "content": "after user"},
+            {
+                "role": "assistant",
+                "content": {"not": "sqlite-bindable but JSONL-safe"},
+            },
+        ]
+
+        store.rewrite_transcript(session_id, replacement)
+
+        after = db.get_messages_as_conversation(session_id)
+        assert [msg["content"] for msg in after] == [
+            "before user",
+            "before assistant",
+        ]