feat: add DisableHTTP2Fallback option

[jpirstin]

Summary

Adds a DisableHTTP2Fallback option to Options struct that, when set to true, prevents the automatic fallback to HTTP/2 client (HTTPClient2) when the HTTP/1.x transport encounters malformed HTTP version errors.

Problem

When callers (e.g., httpx with -pr http11) explicitly configure HTTP/1.1-only transport by setting TLSNextProto to an empty map, the fallback in do.go still switches to HTTPClient2 on certain errors, bypassing the caller's explicit protocol choice. This causes the -pr flag in httpx to not work correctly.

Solution

Added a boolean DisableHTTP2Fallback option. When enabled, the HTTP/2 fallback path in do.go is skipped, and errors are handled according to the normal retry policy.

Changes

• client.go: Added DisableHTTP2Fallback bool to Options struct
• do.go: Guard the HTTP/2 fallback with !c.options.DisableHTTP2Fallback

Fixes projectdiscovery/httpx#2240

[jpirstin]

Hi team — just flagging that CI workflows are showing action_required (first-time contributor gate). Local go test ./... and go vet ./... both pass cleanly. Could a maintainer approve the workflow run when you get a chance? Thanks 🙏

[jpirstin]

Friendly ping — this PR adds a DisableHTTP2Fallback option to prevent automatic HTTP/2→HTTP/1.1 downgrade. All local tests pass. The CI checks show action_required (first-time contributor gate). Could a maintainer approve the workflow run when convenient? Thanks 🙏

[jpirstin]

Checking in on this PR — any feedback or CI gate changes needed to move it forward? Happy to rebase or adjust. Thanks!

[Mzack9999]

The underlying issue (httpx#2240) has been resolved at the httpx level — the HTTP/1.1 protocol preference is now correctly enforced without requiring changes to retryablehttp-go. Closing as the original bounty issue is already complete. Thanks for the contribution.