Isolate authenticated proxy connection pools

[quinnj]

Summary

• assign an opaque identity to each authenticated proxy configuration
• include that identity in HTTP/1 and HTTP/2 connection pool keys
• cover SOCKS5/SOCKS5H and authenticated HTTP CONNECT configurations
• verify credentials never appear in the generated pool key

Root cause

Proxy userinfo is deliberately removed from the normalized proxy URL, but that credential-free URL was also used as the connection pool identity. SOCKS and HTTP CONNECT authentication happens only while establishing the connection, so a later request using a different authenticated proxy configuration could reuse a connection established under the first configuration.

Impact

Authenticated proxy connections are now isolated by configuration without storing usernames, passwords, or reversible credential digests in internal pool keys. Reusing the same ProxyConfig still reuses its connections; independently parsed authenticated configurations are isolated conservatively.

Validation

• full Pkg.test() on Julia 1.12, including all 63 trim-compilation checks
• test/http_client_proxy_tests.jl on Julia 1.10
• git diff --check

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.08%. Comparing base (626dac7) to head (081cb9e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1329   +/-   ##
=======================================
  Coverage   88.08%   88.08%           
=======================================
  Files          30       30           
  Lines       11847    11849    +2     
=======================================
+ Hits        10435    10437    +2     
  Misses       1412     1412           

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