Remove custom entity dependencies from MyrestHelper

Problem/Motivation

The MyrestHelper class heavily depends on custom entity classes like MyentityNode, MyentityTerm, etc., from the myentity module. This prevents myrest from being a standalone module.

Proposed resolution

• Replace custom interface type-hints (e.g., MyentityNodeInterface) with standard Drupal core interfaces (e.g., NodeInterface, TermInterface, ContentEntityInterface).
• Remove use statements for Drupal\myentity\*.

Remaining tasks

Audit methods in MyrestHelper and replace custom types with generic Drupal interfaces.