fix(httpx): enforce http11 for both fallback and h2 probe clients

[wh0amibjm]

Follow-up for projectdiscovery#2414 to address review feedback.

Proposed Changes

• In common/httpx/httpx.go, when Protocol == http11:
  • set retryablehttp.Client.HTTPClient2 = HTTPClient
  • set httpx.client2 = httpx.client.HTTPClient
    This ensures both retry fallback and HTTP/2 probe paths honor explicit HTTP/1.1 mode.
• Keep http2.Transport-backed client2 only for non-http11 modes.
• Extend tests in common/httpx/httpx_protocol_test.go to verify:
  • http11 mode uses the same client for HTTPClient, HTTPClient2, and client2
  • non-http11 mode keeps dedicated HTTP/2 client
  • dialer resources are closed via t.Cleanup.

Proof

Before (origin/dev), with the same test file:

go test ./common/httpx -run TestNew_HTTP11DisablesRetryableHTTP2Fallback -count=1
# FAIL: HTTPClient2 was not same as HTTPClient in http11 mode

After:

go test ./common/httpx -run TestNew_ -count=1
# ok

Checklist:

• PR created against correct branch
• Tests added for behavior
• Local tests passing for affected package