Move config document helpers into their own module (#25110)

## Why

`core/src/config/edit.rs` owns the config edit state machine, but it
also carried the TOML document helper code inline as a nested module.
Moving those helpers into their own file keeps the edit orchestration
easier to scan without changing the config persistence behavior.

## What changed

- Moved the existing `document_helpers` module from
`core/src/config/edit.rs` into
`core/src/config/edit/document_helpers.rs`.
- Added `mod document_helpers;` so the existing `pub(super)` helper API
remains available to the rest of `config::edit`.

## Testing

Not run; this is a refactor-only module extraction with no intended
behavior change.