ai-chat: chat-recovery exhaustion has no terminal-status hydration (disconnected client misses the terminal banner on reconnect) #1645

Summary

In @cloudflare/ai-chat, when chat recovery is exhausted, _exhaustChatRecovery delivers the terminal UX only as a transient broadcast (_broadcastChatMessage with done: true, error: true). It does not durably record the terminal status. A client that is disconnected at the moment recovery gives up (e.g. mid deploy-churn / WS reconnect storm — the exact condition under which recovery exhausts) will never see the terminal banner on reconnect, and the turn appears frozen.

@cloudflare/think does not have this gap: its _exhaustChatRecovery additionally calls _recordTerminalChatStatus("interrupted", …), which persists CHAT_LAST_TERMINAL_KEY, and _buildIdleConnectMessages replays that terminal error to a client on (re)connect. ai-chat has no equivalent.

Scope

This affects all ai-chat recovery exhaustion paths (deploy/eviction exhaustion via _handleInternalFiberRecovery, and the stable-state-timeout give-up), not any single path. It is pre-existing.

Where

• packages/ai-chat/src/index.ts — _exhaustChatRecovery only broadcasts; no durable terminal record.
• Compare packages/think/src/think.ts — _exhaustChatRecovery → _recordTerminalChatStatus("interrupted", …), plus _buildIdleConnectMessages / _pendingTerminalChatResponse for replay on idle connect.

Repro sketch

1. Start a turn on an ai-chat agent with chatRecovery enabled and a custom terminalMessage.
2. Drive recovery to exhaustion while the client socket is closed (e.g. simulate deploy churn so the budget drains while disconnected).
3. Reconnect the client.
4. Expected: the client receives a terminal error message (the configured terminalMessage) on connect.
5. Actual: no terminal frame is replayed; the UI shows no resolution for the abandoned turn.

Proposed fix

Give ai-chat a durable last-terminal record analogous to think's: persist the terminal message/requestId in _exhaustChatRecovery and replay it in the agent's on-connect message build, so an exhausted turn surfaces on reconnect rather than freezing.

Context

Spun out of the chat-recovery stable-state-timeout exhaustion fix (which routes the give-up through onExhausted/terminalMessage like deploy/stall exhaustion). During that review we noted ai-chat's weaker terminal hydration is a separate, pre-existing concern and out of scope for that change.