fix: disable HTTP/2 fallback when -pr http11 is set

[285729101]

Summary

• Override retryablehttp-go's internal HTTPClient2 fallback client with an HTTP/1.1-only transport when -pr http11 is explicitly specified
• This prevents the library from silently falling back to HTTP/2 on malformed HTTP version errors, which was causing the -pr http11 flag to be ignored
• The fix clones the existing HTTP/1.1 transport and sets TLSNextProto to an empty map to fully disable HTTP/2 negotiation on the fallback client

Test plan

• Verify go build ./... compiles successfully
• Verify go test ./common/httpx/... passes
• Manual test: run httpx -u <target> -pr http11 against an HTTP/2 server and confirm only HTTP/1.1 is used
• Manual test: confirm normal (non -pr http11) behavior is unaffected and HTTP/2 fallback still works

/claim #2240

🤖 Generated with Claude Code

[coderabbitai]

Caution

Review failed

An error occurred during the review process. Please try again later.

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[285729101]

@Ice3man543 alternative approach for the HTTP/1.1 flag fix — disables HTTP/2 at the transport level.

[dogancanbakir]

Hi, thanks for your interest in contributing! Just a heads up, we ask contributors to work on 1 active issue at a time (see).

Also, we welcome AI-assisted development, but submissions must be complete, tested, and ready to merge. Please also make sure to fill out the PR template with proof that your changes work.

We're closing this PR along with your other open submissions. Once you're ready, feel free to pick one issue to focus on and resubmit; we'd be happy to review it.

Appreciate your understanding!