This internal function checks whether specified elements exist in a given list
and returns two vectors: elements that exist and elements that do not. When
nested = TRUE and x has results (multi-framework), checks
inside each x$results$<framework>.
check_elements_simple(x, elems, nested = FALSE)A list with:
Character vector of elements that exist.
Character vector of elements that do not exist (top-level only).
Named list (framework -> present elements), when nested.
A list object to check (e.g., `test_results`).
A character vector of element names to check.
If TRUE and x has results, check inside each
framework block. Returns present (flattened unique) and
by_framework (named list: framework -> present elements).