Use CONFIG_NAME constant in ContactFormEditForm

Problem/Motivation

Use the CONFIG_NAME constant in ContactFormEditForm for consistency with core conventions. There are over 30 other examples in core where configuration names are defined as constants, which improves maintainability, reduces typos, and makes refactoring easier.

Steps to reproduce

Proposed resolution

Use CONFIG_NAME constant in ContactFormEditForm

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet