[BUG] TaskCompleted can precede restart-visible API history

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Problem

The public TaskCompleted boundary does not explicitly guarantee that the task's API conversation history is visible to a fresh extension host. A consumer can observe completion, stop or restart the host, and then find no api_conversation_history.json to restore.

This is a candidate additional root cause under #1279. It is separate from the hydration, stale-writer, and resume/eviction hazards addressed by PR #1319.

Evidence

• Mocked E2E run 33292570133 observed TaskCompleted, then immediately found no persisted API conversation history during the restart-persistence create phase.
• The normal mocked E2E suite passed; only the separate restart-persistence phase failed.
• The failure could not be reproduced in 120 local repetitions, so the exact concrete interleaving remains unproven.
• Alloy model PR [Fix] Task history can disappear when users restart after completion #1452 demonstrates that the abstract current contract permits completion and host shutdown before history is restart-visible. A durability-gated contract removes the counterexample within the checked bounds without preventing successful completion.

Contract decision

Choose and document one production contract:

1. TaskCompleted means the task is safe to close or restart, so required UI/API history writes must be restart-visible before the event is emitted; or
2. TaskCompleted means only that completion was accepted, and production exposes a separate persistence-ready acknowledgment for restart-sensitive consumers.

Polling only in the E2E test is not sufficient unless the production event contract explicitly excludes persistence readiness.

Acceptance criteria

• Add a deterministic regression that controls the completion-versus-persistence ordering.
• Define the public completion and persistence-ready semantics in code and tests.
• Ensure a host stopped after the relevant readiness boundary can restore API conversation history.
• Define behavior for failed or indefinitely delayed history writes.
• Preserve successful completion when persistence succeeds.
• Verify the create and fresh-host restart phases end to end.

Non-goals

• Power-loss or filesystem fsync durability beyond visibility to a fresh extension host.
• The separate partial-ask E2E race fixed by PR [Fix] Mocked E2E check no longer times out on streaming previews #1449.