test: deflake request queue fetch-and-handle integration tests

[vdusek]

test_add_and_fetch_requests[single] (and its siblings test_add_requests_in_batches and test_add_non_unique_requests_in_batch) intermittently failed in CI on queue_operation_info.was_already_handled is False.

The Apify request queue has at-least-once delivery. A request can occasionally be fetched again before the platform propagates its handled state, so mark_request_as_handled reports was_already_handled=True (the state before the update) on the duplicate. The single-mode client already does best-effort local dedup and documents this as a low-probability race and API propagation delay, so the queue contract is at-least-once, not exactly-once.

These tests asserted exactly-once delivery on every mark. They now count only first-time handles (if not was_already_handled), which keeps the handled_request_count == desired_request_count invariant while tolerating the occasional duplicate. See #808.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.32%. Comparing base (e8cda0a) to head (2b72228).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #991      +/-   ##
==========================================
- Coverage   90.35%   90.32%   -0.04%     
==========================================
  Files          49       49              
  Lines        3089     3089              
==========================================
- Hits         2791     2790       -1     
- Misses        298      299       +1     

Flag	Coverage Δ
e2e	36.22% <ø> (ø)
integration	57.42% <ø> (-0.04%)	⬇️
unit	78.92% <ø> (ø)

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