feat: add DisableHTTPFallback to respect explicit HTTP/1.1 configuration

[hetbhatt2807]

No description provided.

[neo-by-projectdiscovery-dev]

Neo - PR Security Review

No security issues found

Highlights

• Adds DisableHTTPFallback boolean field to Client struct to control automatic HTTP/1 → HTTP/2 protocol upgrade fallback
• Modifies Do() method to check the flag before attempting HTTPClient2 fallback when HTTP/1.x client receives HTTP/2 responses
• Fallback is triggered by specific Go stdlib error messages indicating protocol mismatch, not user-controllable input

Hardening Notes

• Consider adding DisableHTTPFallback to the Options struct (lines 40-76 in client.go) alongside other configuration options, rather than as a direct Client field, for API consistency and easier configuration during client construction

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[Mzack9999]

The code should be aligned to the left (if you use vscode or cursor it should automatically fmt on file save)

[Mzack9999]

maybe we can control this at package level with something like:

var ENABLE_HTTP2 = true

with default value to true, so that users can disable it via code. What do you think?

[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.