fix: prolong and track shared request queue locks to prevent duplicate processing

[vdusek]

The shared request-queue client (access='shared') locked a batch of up to 25 requests for 180 s in list_and_lock_head, but never tracked, prolonged, or released those locks. As a batch drained slowly, locks expired mid-flight and requests got processed twice - by another consumer that re-locked an expired request, or by a second worker in this process after the request was re-listed. That defeats the class's documented "request locking to prevent duplicate processing" guarantee (double charges in PPE, duplicate dataset items).

Changes:

• Store lock_expires_at per cached head entry (the API returned it, we discarded it).
• Track handed-out requests in _requests_in_progress: skip them when re-listing the head and when fetching, and keep is_finished false while any is outstanding.
• In fetch_next_request, skip candidates whose lock already lapsed, and prolong the lock only when its remaining window is below half the lock time (_MIN_LOCK_TIME_AT_HANDOFF, 90 s). A request handed out from a fresh batch lock needs no prolong, so the common path adds no API write and shared mode stays at 3 writes per request.

Out of scope: handlers outliving the lock window would need a background refresh loop, and deleting leftover head locks on shutdown has no RequestQueueClient hook to attach to (they lapse within the lock TTL anyway).

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.87%. Comparing base (58d6da1) to head (a47a37d).
⚠️ Report is 34 commits behind head on master.

Files with missing lines	Patch %	Lines
...age_clients/_apify/_request_queue_shared_client.py	91.30%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1062      +/-   ##
==========================================
- Coverage   91.89%   91.87%   -0.02%     
==========================================
  Files          51       51              
  Lines        3232     3274      +42     
==========================================
+ Hits         2970     3008      +38     
- Misses        262      266       +4     

Flag	Coverage Δ
e2e	34.97% <6.52%> (-0.37%)	⬇️
integration	57.02% <56.52%> (-0.07%)	⬇️
unit	85.46% <91.30%> (+2.20%)	⬆️

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[vdusek]

Btw. this is a different case than #887