fix: 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: retry classification stays exactly as on master (every impit.HTTPError is transient). The classification fixes follow in #1012.

Split out of #1006. Stacked on #1010.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 97.60479% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.93%. Comparing base (df8a08d) to head (e6f24c5).

Files with missing lines	Patch %	Lines
src/apify_client/_streamed_log.py	75.00%	2 Missing ⚠️
src/apify_client/http_clients/_base.py	98.41%	2 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                       @@
##           fix-http-response-protocol    #1011      +/-   ##
==============================================================
+ Coverage                       94.87%   94.93%   +0.05%     
==============================================================
  Files                              58       58              
  Lines                            5367     5388      +21     
==============================================================
+ Hits                             5092     5115      +23     
+ Misses                            275      273       -2     

Flag	Coverage Δ
integration	92.03% <77.84%> (-0.20%)	⬇️
unit	85.22% <97.00%> (+0.07%)	⬆️

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