feat: Unify the request pipeline across HTTP clients

[vdusek]

Moves call, the retry loop, and the per-attempt request handling from ImpitHttpClient and ImpitHttpClientAsync into HttpClient and HttpClientAsync, leaving Impit as a thin adapter over the transport hooks: send_request, is_retryable_transport_error, is_timeout_error, and close() / aclose() plus context managers. A custom client that overrides call keeps working unchanged and opts out of the shared pipeline. StreamedLog classifies timeouts through the transport-neutral is_timeout_error hook instead of importing Impit.

No behavior change for the built-in clients: the retry policy shipped in #1019 (permanent transport errors fail fast) and #1020 (a failed read of a streamed error body is classified like a failed send and the response is closed) is preserved — the classification now lives in the is_retryable_transport_error hook, and the error-body read handling in the shared pipeline.

Supersedes #1011, rebased onto current master and squashed. Originally split out of #1006.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 97.82609% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.93%. Comparing base (2760aa6) to head (35309d5).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/apify_client/_streamed_log.py	75.00%	2 Missing ⚠️
src/apify_client/http_clients/_base.py	98.60%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1022      +/-   ##
==========================================
+ Coverage   94.87%   94.93%   +0.05%     
==========================================
  Files          58       58              
  Lines        5384     5406      +22     
==========================================
+ Hits         5108     5132      +24     
+ Misses        276      274       -2     

Flag	Coverage Δ
integration	91.52% <66.30%> (-0.14%)	⬇️
unit	85.29% <97.82%> (+0.09%)	⬆️

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