Internal helper. Asserts that x carries the doseprop_stats class,
contains the per-metric stats columns, and carries the attributes
downstream consumers (notably plot_build_doseprop()) depend on. Aborts
with a clear message on failure; returns x invisibly on success.
Usage
validate_doseprop_stats(
x,
arg = rlang::caller_arg(x),
call = rlang::caller_env()
)