Public API `ScalarUDFImpl.return_type` returns internal error in some cases

### Describe the bug

I am trying to upgrade the version of DataFusion that Comet uses. Comet currently relies on `ScalarUDFImpl.return_type` to get the return type of a scalar UDF. This now returns an internal error when called for `DatePartFunc`.

```rust
fn return_type(&self, _arg_types: &[DataType]) -> Result {
internal_err!("return_type_from_exprs shoud be called instead")
}
```

Should we deprecate this function if it is no longer the correct approach for determining UDF return types? It looks like `return_type_from_exprs` should now be used instead?

### To Reproduce

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

Review the public ScalarUDFImpl.return_type API and the DatePartFunc implementation, then compare their behavior with return_type_from_exprs. Confirm which method callers should use and whether the existing API needs deprecation or another compatibility change. Done means the intended public return-type path is established and the DatePartFunc internal error is addressed or clearly documented.

Written by the indexing model from the issue text.