simple_batch_embed raised ValueError when any input exceeded the per-input
token cap, which failed the entire deriver save when a single observation
was over-length. Add on_oversize="truncate": oversize inputs are embedded
from a token-capped prefix (re-encoded until it fits, with a warning),
preserving one vector per input. Default stays "raise" so existing callers
are unchanged. RepresentationManager opts into truncate.
Also add a live embedding test that fails on main (raise / missing kwarg)
and passes once a mixed short+oversize batch survives.
Refs #569
199dae8821