refactor for safety: don't expose FileDescriptor constructor

[mtullsen]

Description

resolves #3835

(the title says it all, one simple commit)

Checklist

• Branch
  • Updated changelog files.
  • Commit sequence broadly makes sense
  • Commits have useful messages
  • The documentation has been properly updated
  • New tests are added if needed and existing tests are updated
  • If serialization changes, user-facing consequences (e.g. replay from genesis) are confirmed to be intentional.
• Pull Request
  • Self-reviewed the diff
  • Useful pull request description at least containing the following information:
    • What does this PR change?
    • Why these changes were needed?
    • How does this affect downstream repositories and/or end-users?
    • Which ticket does this PR close (if any)? If it does, is it linked?
  • Reviewer requested

[coot]

LGTM

[coot]

Stylish haskell complains, you can run it locally with nix .\#stylish-haskell or directly with ./scripts/ci/check-stylish.sh (assuming you have fd and the right version of stylish-haskell installed).