Fix HTTP auto fallback on h2 ALPN rejection

[quinnj]

Summary

• offer http/1.1 alongside h2 for the default high-level protocol=:auto h2 probe
• keep explicit protocol=:h2 strict and continue honoring user-specified ALPN lists verbatim
• update the TLS auto fallback integration test so it covers the default client ALPN path
• drain POST bodies in redirect test servers before close, avoiding Windows TCP resets with current Reseau

Root Cause

protocol=:auto attempted h2 with a default ALPN list of only ["h2"]. Some h1-only servers reject that during TLS with no_application_protocol before HTTP can observe an ALPN mismatch and run the existing h1 fallback.

The Windows CI failures on the first drafts resolved Reseau v1.1.3 and exposed unrelated test harness races: redirect test servers sent Connection: close responses for POST requests without draining the request body, which can reset the TCP connection on Windows.

Validation

• julia --project=. --startup-file=no --history-file=no -e 'using Pkg; Pkg.test(; test_args=["http_integration_tests.jl", "http2_client_tests.jl"])' (test driver ran the full suite)
• julia --project=. --startup-file=no --history-file=no test/http_client_tests.jl
• julia --project=. --startup-file=no --history-file=no test/http_parity_tests.jl
• live HTTP.jl gets 403 from endpoint while curl/Downloads succeeds with same URL and headers #1265 endpoint with default protocol=:auto returns 200 over HTTP/1.1

Refs #1265

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.53%. Comparing base (f9d62af) to head (9efb50b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1269      +/-   ##
==========================================
+ Coverage   84.36%   84.53%   +0.16%     
==========================================
  Files          28       28              
  Lines       10689    10690       +1     
==========================================
+ Hits         9018     9037      +19     
+ Misses       1671     1653      -18     

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