Disable HTTP/2 fallback when using -pr http11 flag

[MAVRICK-1]

Proposed changes

Fixes -pr http11 flag being ignored due to HTTP/2 fallback in retryablehttp-go.

When using -pr http11, httpx disables HTTP/2 at transport level but retryablehttp-go automatically falls back to HTTP/2 client on certain errors. This PR sets DisableHTTP2Fallback = true when http11 mode is enabled.

Depends on: projectdiscovery/retryablehttp-go#522

Proof

Shows HTTP/1.1 only, no HTTP/2 fallback on malformed response errors.

Checklist

• Pull request is created against the dev branch
• All checks passed (lint, unit/integration/tests etc.) with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Fixes #2240
/claim #2240

Summary by CodeRabbit

• Bug Fixes
  • Fixed HTTP protocol handling to properly disable HTTP/2 fallback when HTTP/1.1-only mode is enabled.

[coderabbitai]

Walkthrough

Adds logic to disable HTTP/2 fallback in retryablehttp when HTTP/1.1-only protocol mode is selected in httpx. This ensures that when users explicitly request HTTP/1.1, the client respects that choice instead of automatically falling back to HTTP/2 on connection errors.

Changes

Cohort / File(s)	Summary
HTTP/2 Fallback Suppression common/httpx/httpx.go	Adds conditional to disable HTTP/2 fallback in retryablehttp when protocol is set to "http11", preventing unintended protocol switches when HTTP/1.1-only mode is explicitly configured.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 A rabbit hops through protocols fair,
Where HTTP/1.1 rules the air,
No more fallback tricks to play,
Just one true path to save the day! 🚀

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely summarizes the main change: disabling HTTP/2 fallback when the -pr http11 flag is used.
Linked Issues check	✅ Passed	The PR successfully implements the required fix by setting DisableHTTP2Fallback = true when HTTP/1.1 mode is enabled, directly addressing issue #2240.
Out of Scope Changes check	✅ Passed	All changes are focused on disabling HTTP/2 fallback for HTTP/1.1 mode; no unrelated modifications were introduced.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

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

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