[codex] Recover ACP runtime errors from snapshots

[ijry]

Summary

This PR makes ACP runtime errors recoverable from the live session snapshot. The main user-visible case is a client that was offline or outside the replay window when the server emitted an AcpEvent::Error, such as:

ACP protocol error: Internal error: unexpected status 403 Forbidden ... rolling spend limit exceeded

Before this change, a live web client could show that error because it received the realtime event, while MCode/mobile or any reconnecting client could miss it and hydrate from a snapshot with no error detail.

Root Cause

SessionState already captured the latest AcpEvent::Error in last_error, but SessionState::to_snapshot() intentionally omitted it. That made the error delivery path realtime-only:

• Online clients received AcpEvent::Error and updated ConnectionState.error.
• Reconnecting/offline clients hydrated through LiveSessionSnapshot.
• The snapshot did not include last_error, so clients that missed the realtime event had no durable source for the latest ACP runtime failure.

This is why a CodeG web session could show the spend-limit 403 under the input box, but an MCode/mobile client could show nothing or incomplete state after reconnect.

Changes

• Expose SessionState.last_error as optional LiveSessionSnapshot.last_error.
• Keep the field backward-compatible with serde(default, skip_serializing_if = "Option::is_none"), so normal snapshots omit it and older clients can ignore it.
• Clear last_error when a new prompt enters Prompting, matching the existing frontend live reducer behavior where a new prompt clears the visible error scope.
• Mirror the wire type in TypeScript as SessionLastError and LiveSessionSnapshot.last_error.
• Denormalize snapshot last_error.message into SnapshotPatch.lastError.
• Hydrate ConnectionState.error from fresh snapshots; for stale snapshots, only fill the error if the current live state does not already have a newer error.
• Update the connection-context test mock to include the now-complete SnapshotPatch shape.

Compatibility / Client Guidance

The new field is optional and omitted when absent:

last_error?: { message: string; code?: string | null } | null

Native iOS/Android/MCode clients should treat last_error?.message as the snapshot recovery value for the same UI surface that currently consumes realtime ACP error events. The field is not an event replay replacement; it is the latest unresolved runtime error that lets a reconnecting client recover what it missed.

Clients should also preserve their realtime ordering rules: if a local state already observed a newer realtime error, an older/stale snapshot should not overwrite it.

Validation

• cargo test --no-default-features --features test-utils --lib snapshot_carries_last_error_and_clears_on_next_prompt
• pnpm test src/lib/snapshot-denormalize.test.ts src/contexts/acp-connections-context.test.tsx
• pnpm exec eslint src/lib/snapshot-denormalize.ts src/lib/snapshot-denormalize.test.ts src/contexts/acp-connections-context.tsx src/contexts/acp-connections-context.test.tsx src/lib/types.ts
• git diff --check

Note: the default cargo test --features test-utils path on Windows attempted to build the Tauri desktop target and failed before tests because ../out static assets were not present. The shared/server-side targeted command above avoids the desktop build-script asset requirement and exercises this ACP snapshot logic directly.

[ijry]

这种acp报错似乎如果其他端没实时在线获取到，比如App端掉线，过了很久才打开查看，看不到这个报错。需要写进to_snapshot。