MessageService.answer() releases its per-session lock in an outer finally
(MessageService.java:1218) that mutation testing showed was covered but
unasserted: removing that line left all 1750 existing tests green, because
every existing test on this path checks answer()'s return value, never that
the lock it took is actually reacquirable afterward. If it leaked, a session
would be wedged forever with no exception and no log line.
Adds four tests, one per exit of answer() (normal REPLIED reply,
TIMED_OUT_WORKING, ExecutionException rethrow, InterruptedException
rethrow), each proving the lock is reacquirable via a bounded (300ms)
follow-up send on the same session rather than merely checking answer()'s
own outcome. No production change.
Dai Ha