Improve type safety incase of readonly objects

To avoid such logical issues for future we should fix the types.

_Originally posted by @nazarhussain in https://github.com/ChainSafe/lodestar/pull/6774#discussion_r1599953114_

The issue names no files, tests, or entry points. Start by reading the referenced discussion in pull request #6774 to identify the readonly-object type-safety concern, then establish the affected scope and an agreed completion criterion before making changes.

Written by the indexing model from the issue text.