fix: emit completed messages when providers omit deltas

[quabug]

Some OpenAI-compatible Responses providers, including Volcengine Ark, deliver final text without item/agentMessage/delta. Codex app-server still emits item/started and item/completed for an agentMessage carrying the full text, but codex-acp currently ignores both. ACP clients receive reasoning and a successful end_turn with no final message.

Track agent-message item IDs that emitted deltas. On item/completed, retain the current no-op for those streamed messages; otherwise emit the completed item's non-empty text once, including its phase metadata.

Validation:

• npm run typecheck
• npx vitest run --no-file-parallelism src/__tests__/CodexACPAgent/agent-message-events.test.ts
• npm run build
• Real Ark deepseek-v4-flash ACP turn: before this change, only the thought event arrived; after this change, the client received exactly one final agent_message_chunk containing ARK_ACP_RAW_OK.