fix: surface async user input events

[jbulpitt]

Summary

• surface completed delivery: "async" agent-message questions through ACP _meta.codex.asyncUserInput
• retain the app-server thread/turn/item correlations and structured question/options payload
• suppress repeated completed text when the same item already emitted an agent-message delta
• emit completed-only question text for clients that do not yet consume the extension

OpenAI Codex's request_user_input_async returns {"accepted":true} immediately. Its actual question arrives later as an item/started and item/completed agentMessage with delivery:"async" and questions; a reply is an ordinary new user message on the same thread, not a response to a pending JSON-RPC request. The current handler drops both item notifications, so ACP clients cannot render or route the question.

Verification

• npm run typecheck
• vitest run src/__tests__/CodexACPAgent/agent-message-events.test.ts --no-file-parallelism — 3/3
• mutation proof: removing the production handler change makes both new cases fail; restoring it passes

The patch changes no blocking item/tool/requestUserInput request/response behavior.

Downstream context: jbulpitt/seam-acp#297