Fix first chat response transport

[ComputelessComputer]

Recreate chat sessions when transport becomes ready and persist first-send responses against the newly created chat group.

---

Note

Medium Risk
Touches core chat persistence, SDK lifecycle, and message sync; regressions could lose messages, show stale UI, or attach replies to the wrong group.

Overview
Fixes chat sessions that started before the model transport was ready, and assistant rows that were saved under the wrong group when the first message created a new chat group.

ChatSession now builds an AI SDK Chat instance (recreated when transport changes) and wires useChat({ chat }), with a stub transport until the model is ready. sendMessage accepts optional { chatGroupId }; that id (or the user row’s persisted chat_group_id) is used in onFinish so assistant replies land on the correct group, including overlapping in-flight sends. When status is ready, persisted TinyBase messages sync into the SDK without clobbering streaming UI state. handleSendMessage passes the group id used when creating a group on first send.

Types in ChatContent and expanded session-provider tests document the new behavior.

^{Reviewed by Cursor Bugbot for commit 7d7b284. Bugbot is set up for automated code reviews on this repo. Configure here.}

[netlify]

✅ Deploy Preview for old-char canceled.

Name	Link
🔨 Latest commit	7d7b284
🔍 Latest deploy log	https://app.netlify.com/projects/old-char/deploys/6a3e555064800b0008618074

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

There are 2 total unresolved issues (including 1 from previous review).

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 2ec9ccf. Configure here.}