feat: Add HTTPX-based HTTP client

[vdusek]

Adds HttpxHttpClient / HttpxHttpClientAsync as built-in alternatives to the default Impit transport, behind an optional extra: pip install "apify-client[httpx]". Impit stays the default.

The transport is HTTPX2, Pydantic's maintained continuation of HTTPX (stable HTTPX has been parked at 0.28.1 since Dec 2024). The module imports it as httpx, so the client keeps the plain Httpx* naming. It verifies TLS via the OS trust store (truststore) rather than certifi.

The adapters plug into the shared request pipeline from #1022 and implement only the transport hooks (send_request, is_retryable_transport_error, is_timeout_error, close() / aclose()), applying the same retry policy as Impit: every error from the library's own hierarchy is transient except the permanently-failing types. A response event hook keeps the shared cookie jar from leaking server cookies into later API requests. Unit tests run the built-in client matrix (Impit + HTTPX) through the parametrized fixtures.

Note that HTTPX applies timeouts per socket operation, not to the request as a whole, so a slowly-arriving body can outlast the requested timeout. Impit enforces a whole-request deadline. Both class docstrings record this.

Stack: #1022 (merged) → #1004 (this) → #1013 (docs — its apify-client[httpx] install lines now match; only the conda-forge::httpx line still points at the stale package instead of httpx2).

✍️ Drafted by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 98.43750% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.25%. Comparing base (514848f) to head (c8b3628).

Files with missing lines	Patch %	Lines
src/apify_client/http_clients/__init__.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1004      +/-   ##
==========================================
+ Coverage   95.23%   95.25%   +0.01%     
==========================================
  Files          58       59       +1     
  Lines        5436     5499      +63     
==========================================
+ Hits         5177     5238      +61     
- Misses        259      261       +2     

Flag	Coverage Δ
integration	92.19% <90.62%> (+0.12%)	⬆️
unit	87.34% <98.43%> (+0.12%)	⬆️

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[Pijukatel]

Two minor comments

[Pijukatel]

Maybe share the default and only extend. Less duplication and it is clear from first look what is the optional

__all__ = ...

if _httpx_import.available:
    __all__.extend(...)

[Pijukatel]

Small concern. The default most common constructor will no longer be well covered by the integration tests.

ApifyClient(api_token, api_url=api_url)
ApifyClientAsync(api_token, api_url=api_url)