[Bug]: Provider finish_reason network_error hides revert button

[ImBIOS]

Description

When the provider finishes with finish_reason: network_error the last user message row shows only the copy button — the revert/undo affordance is missing. Users expect to be able to revert the failed turn (to retry or remove it), especially after transient network failures.

Screenshot/behavior: input I last gave shows only copy, no revert arrow.

Root cause

• Failed turns still create a checkpoint (CheckpointReactor.ts:355 status: error, thread.turn.diff.complete), but they often have no assistant_message or a synthetic assistant:${turnId} id that never appears in timelineEntries.
• ChatView.tsx:2620 derives revertTurnCountByUserMessageId only by scanning forward to the next assistant message and looking up turnDiffSummaryByAssistantMessageId. When the assistant message is missing/synthetic the lookup misses -> canRevertAgentWork false -> MessagesTimeline.tsx:995 hides the revert button.

Evidence:

• apps/server/src/orchestration/Layers/CheckpointReactor.ts:355,65 creates error checkpoint.
• apps/web/src/components/ChatView.tsx:2620-2651 direct scan misses.
• apps/web/src/components/chat/MessagesTimeline.tsx:995,1068 only renders copy.

Expected

After any terminal turn, including network_error/status:error/missing, the preceding user message must expose the revert button (disabled only when isRevertingCheckpoint or isWorking).

Patch

Managed via fh as fix-chat-show-revert-after-network-error-vq3yckjy:

• Keep existing assistant-message scan, add fallback: map nth user message to nth checkpoint sorted by completedAt (1:1 user->turn) when direct scan finds nothing. Covers synthetic ids and zero-message failed turns.
• Change: apps/web/src/components/ChatView.tsx:2620 + fallback block.

Verify:

pnpm exec vp test run apps/web/src/components/ChatView.logic.test.ts
pnpm exec vp test run apps/web/src/components/chat/MessagesTimeline.logic.test.ts

Manual: trigger Provider finish_reason: network_error (e.g. kill network mid-turn), observe last user bubble now shows undo icon next to copy.

Tracking

• Patch: fix-chat-show-revert-after-network-error-vq3yckjy in ImBIOS/.forkhub
• Source branch: fix-chat-show-revert-after-network-error