fix(curl-impersonate): treat a refused CONNECT tunnel as a proxy error

[vdusek]

curl-cffi 0.16.0 broke two things in CurlImpersonateHttpClient, which is why #2107 capped it. This fixes both and drops the cap.

1. Bundled libcurl went 8.15.0 -> 8.21.0, so a refused CONNECT tunnel is now CURLE_COULDNT_CONNECT with CONNECT tunnel failed instead of CURLE_RECV_ERROR. curl_cffi maps only the latter to CurlProxyError, so _is_proxy_error stopped recognizing it and sessions were not rotated. The needle lives in _curl_impersonate.py rather than the shared ROTATE_PROXY_ERRORS, which mirrors the JS Crawlee list - impit and httpx never produce libcurl messages.

2. The session cookie write path moved to Cookies.update_cookies_from_curl_changes, leaving the _EmptyCookies override inert. Isolation held (get_cookies_for_curl is still overridden, so nothing was ever sent), but every response cookie accumulated in the jar of the per-proxy session cached for the client's lifetime. Measured with the same 25-request repro: 25 cookies on 0.16.0, 0 on 0.15.0.

The existing test_crawl_with_proxy_disabled[curl] and test_send_request_with_proxy_disabled[curl] cover the first; test_internal_session_cookie_jar_stays_empty is new for the second.

Closes: #2111

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.56%. Comparing base (463806f) to head (67d4f4b).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2109   +/-   ##
=======================================
  Coverage   93.55%   93.56%           
=======================================
  Files         181      181           
  Lines       12590    12595    +5     
=======================================
+ Hits        11779    11784    +5     
  Misses        811      811           

Flag	Coverage Δ
unit	93.56% <100.00%> (+<0.01%)	⬆️

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[Mantisus]

LGTM