Expose Codex agent message phases in ACP chunks

[nikita-ashihmin]

Summary

• add Codex message phase metadata under _meta.codex.phase for streamed agent_message_chunk updates
• preserve phase metadata when replaying thread history and response-item fallback history
• add coverage for commentary vs final_answer message chunks and fallback phase preservation

Wire Format Example

Before this change, visible assistant text and mid-turn commentary both arrived as the same ACP block type with no phase metadata. A client could see that this was an agent_message_chunk, but could not tell whether it should be rendered inside a Working/Worked group or as the final answer:

{
  "sessionUpdate": "agent_message_chunk",
  "messageId": "commentary-message",
  "content": {
    "type": "text",
    "text": "Checking the relevant event mapping."
  }
}

After this change, the block type is still the standard ACP agent_message_chunk, but Codex-specific phase metadata is attached under _meta.codex.phase:

{
  "sessionUpdate": "agent_message_chunk",
  "messageId": "commentary-message",
  "content": {
    "type": "text",
    "text": "Checking the relevant event mapping."
  },
  "_meta": {
    "codex": {
      "phase": "commentary"
    }
  }
}

Final answer text uses the same ACP block type, with phase: "final_answer":

{
  "sessionUpdate": "agent_message_chunk",
  "messageId": "final-message",
  "content": {
    "type": "text",
    "text": "Yes, here is the answer."
  },
  "_meta": {
    "codex": {
      "phase": "final_answer"
    }
  }
}

This lets clients keep compatibility with ACP while routing commentary chunks into a Working/Worked group and final_answer chunks into the visible assistant answer.

Tests

• npm test -- src/__tests__/CodexACPAgent/agent-message-events.test.ts src/__tests__/CodexACPAgent/response-item-history-fallback.test.ts
• npm run typecheck
• npm test