The current implementation of `forward_list::resize` does not meet the
strong exception safety guarantee required by [forward.list.modifiers]:
If an exception is thrown by any of these member functions there is no
effect on the container.
This patch refactors `resize()` to provide strong exception safety and
introduces additional tests to validate the strong exception guarantees
for other `forward_list` modifiers.
Fixes#118366.
ec5610c4a2