test: deflake is_finished assertions in shared request queue mode

[vdusek]

test_request_queue_is_finished_and_is_empty[shared] failed on master
(run 31375518173)
with assert not True.

In shared access mode is_finished() answers from the queue head listing, which is eventually
consistent: right after a committed add_request the head can still come back empty with no locked
requests, so the queue briefly reports itself as finished. The test asserted that negative condition
with a single immediate read, while every positive assertion around it already polls.

Both negative is_finished assertions now poll with poll_until_condition and the mode-appropriate
rq_poll_timeout, so they tolerate the propagation window without losing their meaning - in single
mode the timeout is 0, so it stays a single strict read, and a queue that never stops reporting
itself as finished still fails.

Verified by injecting the lag deterministically (first head listing after each add forced empty):
the current test reproduces the exact CI failure, the updated one passes 25/25 runs under the same
condition. The whole module also passes under --numprocesses=auto.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.27%. Comparing base (3212e1c) to head (c763e75).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1087      +/-   ##
==========================================
+ Coverage   92.24%   92.27%   +0.03%     
==========================================
  Files          51       51              
  Lines        3249     3249              
==========================================
+ Hits         2997     2998       +1     
+ Misses        252      251       -1     

Flag	Coverage Δ
e2e	35.24% <ø> (ø)
integration	56.69% <ø> (+0.03%)	⬆️
unit	84.51% <ø> (ø)

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