sum_of() does not appear to work with tidyselect

```
mtcars %>% mutate(x := sum_of(ends_with("t")))
# Error: Problem with `mutate()` input `x`.
# x `ends_with()` must be used within a *selecting* function.
# ℹ See .
# ℹ Input `x` is `sum_of(ends_with("t"))`.
```

Start by reproducing the mtcars example from the issue and inspect the sum_of() entry point and its interaction with tidyselect. The issue names no files or tests, so locate the relevant implementation and existing selection-context coverage first. Done means the shown mutate() expression works with ends_with("t") without the selecting-context error.

Written by the indexing model from the issue text.