Migrate rooms.nameExists endpoint to new API format

[CodeMatrix1]

Problem

The rooms.nameExists endpoint currently uses the deprecated API.v1.addRoute method.

Rocket.Chat is migrating REST endpoints to the new API format (API.v1.get, API.v1.post, etc.) which provides:

• built-in AJV schema validation
• automatic OpenAPI generation
• improved type safety
• consistent endpoint structure

Goal

Migrate the rooms.nameExists endpoint to the new API format.