Custom Actions can be replaced without warning

### Bug description

Custom Actions are apparently registered by name, so if you create one with a name matching an existing one then it (silently!) replaces the old one:

https://github.com/user-attachments/assets/f1cd7da3-100d-44b7-b91a-55e58feebdc7

### Steps to reproduce

1. Create a Custom Action
2. Create another one with the same name and type, but different features
3. On save, only the new one exists (the old one has been replaced)

### Primary pain point(s)

It's unexpected for a name conflict to cause an instance replacement without warning.

Ideally choosing an existing name when it is in use elsewhere should be treated as invalid input, with a warning like "Action names must be unique - please choose a different one."

### Additional context

_No response_

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.