chore: extract sanitizeField from sanitizeFields (#16133)

Extracts per-field sanitization logic into a standalone `sanitizeField`
function that can be called independently. The `sanitizeFields` function
now delegates to `sanitizeField` in a loop, maintaining identical
behavior while exposing the core logic.

This enables sanitizing individual fields after initial config
sanitization, which is needed for the orderable optimization in the
follow-up PR.

Exports `sanitizeField` and `SanitizeFieldArgs` from the payload
package.