fix: retry unprocessed requests in RequestQueue.add_request

[vdusek]

Description

RequestQueue.add_request (singular) made a single best-effort add_batch_of_requests([request]) call and, if the storage client returned the request as unprocessed, just logged a warning and returned None — silently dropping the request. add_requests (plural) already retries unprocessed requests via _process_batch, so the two adds had inconsistent durability against best-effort backends (e.g. the Apify platform's batch_add_requests endpoint, which may legitimately return a request as unprocessed).

This makes a single add as durable as a batched one:

• _process_batch now returns an AddRequestsResponse aggregating the requests processed across all attempts plus any still unprocessed after the retries are exhausted (it previously returned None).
• add_request routes its single request through _process_batch and returns processed_requests[0], returning None only after retries are exhausted.

The ProcessedRequest | None return contract and the blocking semantics are preserved (for one request, add_requests already runs the first batch synchronously), and the retry is safe because adds are idempotent by unique_key.

Note: on the failure path add_request is now blocking-with-backoff (worst case a few seconds of retry sleeps before returning None), where it previously returned immediately. That is the intended trade — durability over a fast silent drop.

Issues

• Closes: RequestQueue.add_request does not retry unprocessed requests (inconsistent with add_requests) #1975
• Surfaced as an intermittent e2e flake in test: deflake request queue aliases e2e test apify-sdk-python#1000

Testing

• Added test_add_request_retries_unprocessed (a request reported unprocessed on the first attempt is retried and survives) and test_add_request_returns_none_after_exhausting_retries (stays unprocessed across all attempts → returns None, 1 initial + 5 retries). Both fail on master and pass with this change.
• uv run poe lint, uv run poe type-check, and the tests/unit/storages/test_request_queue.py suite all pass.

Checklist

• CI passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.02%. Comparing base (970f93b) to head (5d551c2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1976      +/-   ##
==========================================
+ Coverage   92.94%   93.02%   +0.07%     
==========================================
  Files         167      167              
  Lines       11737    11740       +3     
==========================================
+ Hits        10909    10921      +12     
+ Misses        828      819       -9     

Flag	Coverage Δ
unit	93.02% <100.00%> (+0.07%)	⬆️

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