chore: refactor FileManager initialization and introduce with_reader constructor

[ghost]

Description

FileManager Refactoring: The FileManager::new function was refactored. Previously, it required a Box as a parameter for file reading. Now, it has been split into two methods:

• FileManager::new, which no longer takes a Box and uses std::fs::read_to_string by default.
• FileManager::with_reader, which takes a Box to allow custom file reading.

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[kevaundray]

This is superceded in favour of #3760