Preserve prepared requests on HTTP/2 fallback

[quinnj]

Summary

• Reuse the prepared request when automatic HTTP/2 negotiation falls back to HTTP/1.1.
• Preserve trace-time header and body mutations, including cloud request signatures.
• Add a TLS regression test that signs the request from RequestEvent and verifies that the HTTP/1.1 server receives the signature.

Root cause

_do_incoming! created and traced a send request before it tried HTTP/2. When ALPN selected HTTP/1.1, the fallback path discarded that prepared request and copied the unchanged source request again. The second copy lost all changes made after the first copy, including trace-time signing and cookie insertion.

The ALPN failure happens during connection setup. No HTTP request body has been sent or consumed at that point. The fallback can safely send the already prepared request.

User impact

Callers can keep HTTP.jl's default protocol=:auto behavior. Authenticated clients no longer need to force HTTP/1.1 to keep trace-time request signatures on fallback.

Validation

• Confirmed the new regression test fails on HTTP.jl 2.6.0 because the server receives no Authorization header.
• Confirmed the regression test passes after the fix.
• Ran Pkg.test() on Julia 1.12.6. The full HTTP.jl suite passed, including all 63 trim-compile checks.
• GitHub Actions CI run 1675 is green on stable and prerelease Julia across Linux and Windows, stable Julia on macOS ARM64, and documentation. The macOS job passed on rerun after an unrelated raw HTTP/1 first-byte timeout.

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.32%. Comparing base (ed8303c) to head (a31ef60).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1341      +/-   ##
==========================================
+ Coverage   88.30%   88.32%   +0.01%     
==========================================
  Files          30       30              
  Lines       11916    11915       -1     
==========================================
+ Hits        10523    10524       +1     
+ Misses       1393     1391       -2     

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