[App server] add mcp tool call item started/completed events

[celia-oai]

this PR does two things:

1. refactor apply_bespoke_event_handling into a separate file as it's getting kind of long;
2. add mcp tool call item/started and item/completed events. To roll out app server events asap we didn't properly migrate mcp core events to use TurnItem for mcp tool calls - this will be a follow-up PR.

real events generated in log:

{
  "method": "codex/event/mcp_tool_call_end",
  "params": {
    "conversationId": "019a8021-26af-7c20-83db-21ca81e44d68",
    "id": "0",
    "msg": {
      "call_id": "call_7EjRQkD9HnfyMWf7tGrT9FKA",
      "duration": {
        "nanos": 92708,
        "secs": 0
      },
      "invocation": {
        "arguments": {
          "server": ""
        },
        "server": "codex",
        "tool": "list_mcp_resources"
      },
      "result": {
        "Ok": {
          "content": [
            {
              "text": "{\"resources\":[]}",
              "type": "text"
            }
          ],
          "isError": false
        }
      },
      "type": "mcp_tool_call_end"
    }
  }
}

{
  "method": "item/completed",
  "params": {
    "item": {
      "arguments": {
        "server": ""
      },
      "error": null,
      "id": "call_7EjRQkD9HnfyMWf7tGrT9FKA",
      "result": {
        "content": [
          {
            "text": "{\"resources\":[]}",
            "type": "text"
          }
        ],
        "structuredContent": null
      },
      "server": "codex",
      "status": "completed",
      "tool": "list_mcp_resources",
      "type": "mcpToolCall"
    }
  }
}

[owenlin0]

nice! looks straightforward 🙌

[owenlin0]

nit: can we call this type JsonValue?