test: poll the first fetch in the shared RQ in-progress test

[vdusek]

test_shared_is_finished_false_while_request_in_progress failed on an unrelated PR's CI
(run 32959613011) with
assert None is not None: its first fetch_next_request() came back empty.

In shared mode that is expected, not a client defect. The fetch goes through list_and_lock_head, which is
eventually consistent, so a just-committed request can be missing from the head for a moment (#808). The crawler
tolerates it because is_finished() still reports False while the request is unhandled, so it retries instead of
shutting down.

Every other shared-mode read in the module already polls with poll_until_condition, as the note at the top of the
file prescribes; this one call site did not. It now polls with the same timeout=30, backoff_factor=2 used by the
other shared-only tests in the file (the rq_poll_timeout fixture is unavailable here, since the test opens its own
queue instead of using the parametrized fixture).

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.68%. Comparing base (5a0a752) to head (67f57be).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1105      +/-   ##
==========================================
+ Coverage   92.65%   92.68%   +0.02%     
==========================================
  Files          51       51              
  Lines        3445     3445              
==========================================
+ Hits         3192     3193       +1     
+ Misses        253      252       -1     

Flag	Coverage Δ
e2e	33.43% <ø> (ø)
integration	54.51% <ø> (+0.02%)	⬆️
unit	89.17% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.