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.

Stack: #1022 (this) → #1004 (HTTPX client) → #1013 (docs).

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 97.88360% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.96%. Comparing base (139a426) to head (8399dd8).
⚠️ Report is 2 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	98.64%	2 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## master #1022 +/- ##
==========================================
+ Coverage 94.90% 94.96% +0.05% 
==========================================
Files 58 58 Lines 5418 5438 +20 ==========================================
+ Hits 5142 5164 +22 + Misses 276 274 -2 

Flag	Coverage Δ
integration	91.76% <71.95%> (+0.12%)	⬆️
unit	85.82% <97.88%> (+0.08%)	⬆️

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[Pijukatel]

I see your PR attracted some of my comments to pre-existing code. Well, that is what you get for moving the code around :-D