Add MCP tool call item to codex exec (#4481)

No arguments/results for now.
```
{
  "type": "item.started",
  "item": {
    "id": "item_1",
    "item_type": "mcp_tool_call",
    "server": "github",
    "tool": "search_issues",
    "status": "in_progress"
  }
}
{
  "type": "item.completed",
  "item": {
    "id": "item_1",
    "item_type": "mcp_tool_call",
    "server": "github",
    "tool": "search_issues",
    "status": "completed"
  }
}
```

codex-rs/exec/Cargo.toml

@@ -58,3 +58,4 @@ tempfile = { workspace = true }
 uuid = { workspace = true }
 walkdir = { workspace = true }
 wiremock = { workspace = true }
+mcp-types = { workspace = true }