[App server] add mcp tool call item started/completed events (#6642)

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"
    }
  }
}
```

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -640,7 +640,7 @@ pub enum McpToolCallStatus {
 #[ts(export_to = "v2/")]
 pub struct McpToolCallResult {
     pub content: Vec<McpContentBlock>,
-    pub structured_content: JsonValue,
+    pub structured_content: Option<JsonValue>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]