[CahidArda]

Fixes point 1 of Sertug's DX-3182 feedback, ahead of the catalog PR.

Problem

sync_turn → save_chat read the whole chat document (JSON.GET) and wrote it back (JSON.SET) on every turn: all messages plus the extracted userMessages / modelMessages text, about 2x the transcript. One long-running session (Sertug's Bot Chat: ~0.9 MB of text after 3 weeks) sends ~1.7 MB per turn and would stop saving silently once it passes Upstash's 10 MB request limit.

Fix

• ChatHistory.append_messages(session_id, messages, user_id=...): one atomic Lua script (#!lua flags=allow-key-locking, one key) that JSON.ARRAPPENDs the messages, JSON.STRAPPENDs the new text to both search fields (with the \n separator only when the field already has text), and sets updatedAt. Text is JSON-encoded in Python (dumps, like JSON.stringify), never by cjson. It returns 0 and writes nothing when the chat doesn't exist, so the provider falls back to a full save_chat for the first turn.
• The provider appends each turn and only reads and rewrites the whole transcript for /undo. The in-memory transcript cache is gone (it only served /undo, and grew with the session).
• The document ends up identical to a full save, so the layout stays shared with the TS ChatHistory. The TS saveChat still does a full replace; matching it there is a separate change.

Also fixed: /undo after a restart

RedisJSON returns a whole-number stamp like 1000.0 as 1000 (verified live). _cut_by_time only accepted float, so a transcript read back from Redis fell back to the content cut, and of two identical turns the wrong one could be removed. The in-memory cache used to hide this within one process. Any number now counts.

Bumps upstash-hermes-memory to 0.2.2. Merging publishes it to PyPI.

Tests

• Live Redis: appending message by message ends exactly where a full save ends (unicode, quotes, /, a whitespace-only message, parts messages, a side's first text); createdAt/title kept; search finds the appended text; append to a missing chat writes nothing.
• Provider: once the chat exists, a turn sends a single EVAL with only the new messages (no JSON.GET/JSON.SET), even with a 5 KB earlier turn.
• /undo with stamps read back as ints.
• Mutation-checked: going back to the full rewrite, or to the float-only check, fails 3 tests.
• uv run pytest: 72 passed against live Redis. ruff, prettier, sync-version --check, uv sync --locked clean.

Not run: the Hermes e2e (e2e/hermes_e2e.py, needs a real Hermes and a model key).