When moving importers from Python to C++ validation was removed
(from PLY) & disabled by default (STL & OBJ) which re-introduces
crashes reported by users such as #31835.
Enable validation by default because crashes caused by imported data
should be avoided, especially since the crashes may happen later when
users enter edit-mode or run certain editing operations.
This does slow down importing, from testing a 236MB .OBJ it takes around
twice as long to import (~1.5 to ~3 seconds). Although validation can
be optimized to reduce the overhead ad well as run in parallel for
importers that load multiple objects.
The defaults for USD and Alembic remain unchanged since this was never
enabled by default, although we could consider enabling these as well.
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