Make `taskDefName` to take precedence over `name` in `SIMPLE` tasks

### The feature, motivation and pitch

Current setup with having to register task definition with given name and the using it as a `name` in `SIMPLE` task is counterintuitive.

But I can see we have `taskDefName` as base task field. how about using it as first lookup field for looking for task type?

### Alternatives

Creating new variable for simple task that will be explicitly stated as it's subType?

### Additional context

_No response_

Start by tracing how SIMPLE tasks resolve their task type from name and taskDefName, then inspect existing task-resolution tests. Done means taskDefName is checked first while the current name-based behavior remains available when it is absent; add or update coverage for both paths.

Written by the indexing model from the issue text.