The original tests replicated the try/except/cancel/raise pattern inline with
a mocked future, which tested Python's try/except semantics rather than the
scheduler's behavior. Rewrite them to invoke _deliver_result and
_send_media_via_adapter end-to-end with a real concurrent.futures.Future
whose .result() raises TimeoutError.
Mutation-verified: both tests fail when the try/except wrappers are removed
from cron/scheduler.py, pass with them in place.
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