fix: Retry on all impit.HTTPError exceptions, not just specific subclasses

[vdusek]

Summary

• Broadened is_retryable_error() to catch impit.HTTPError (base class) instead of only impit.NetworkError, impit.TimeoutException, and impit.RemoteProtocolError
• Some transient errors (e.g. body decode failures like "unexpected EOF during chunk size line") are raised by impit as bare HTTPError instances that don't match any of the specific subclasses, causing requests to fail without retrying
• HTTP status code errors are already handled separately in _make_request based on response status codes, so this change only affects transport-level failures

Triggered by: flaky e2e test failure in apify-sdk-python where wait_for_finish failed with:

impit.HTTPError: The internal HTTP library has thrown an error:
reqwest::Error {
    kind: Decode,
    source: reqwest::Error {
        kind: Body,
        source: hyper::Error(
            Body,
            Custom {
                kind: UnexpectedEof,
                error: "unexpected EOF during chunk size line",
            },
        ),
    },
}

Test plan

• Added parametrized unit tests for is_retryable_error covering all impit exception types (retryable) and generic exceptions (non-retryable)
• Added integration-style test test_retry_on_http_error_async_client that verifies bare HTTPError is actually retried by the HTTP client
• All 115 unit tests pass
• Pre-commit hooks pass (lint, type-check, docstrings)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.18%. Comparing base (e7ee8b3) to head (7d65893).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #624      +/-   ##
==========================================
+ Coverage   76.02%   76.18%   +0.16%     
==========================================
  Files          42       42              
  Lines        2482     2482              
==========================================
+ Hits         1887     1891       +4     
+ Misses        595      591       -4     

Flag	Coverage Δ
integration	68.77% <ø> (ø)
unit	64.82% <ø> (+0.16%)	⬆️

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[vdusek]

@barjin Could you please confirm that all impit.HTTPError should be retried?

[barjin]

impit is an HTTP client, all of its errors are HTTPError subclasses 😅 But yes, given the error typing might have some rough edges, this change is sound.